diff --git a/README.rst b/README.rst
index 0eab496..7bdae7b 100644
--- a/README.rst
+++ b/README.rst
@@ -20,7 +20,7 @@ A **G**\ enetic **A**\ lgorithm **F**\ ramework in py\ **T**\ hon
     :target: https://www.python.org/downloads/release/python-351/
     :alt: platform
 
-.. image:: https://img.shields.io/badge/pypi-v0.6.0-blue.svg
+.. image:: https://img.shields.io/badge/pypi-v0.5.5-blue.svg
     :target: https://pypi.python.org/pypi/gaft/
     :alt: versions
 
diff --git a/gaft/__init__.py b/gaft/__init__.py
index 78aabb6..a217d3e 100644
--- a/gaft/__init__.py
+++ b/gaft/__init__.py
@@ -3,7 +3,7 @@
 
 from .engine import GAEngine
 
-__version__ = '0.6.0'
+__version__ = '0.5.5'
 __author__ = 'ShaoZhengjiang <shaozhengjiang@gmail.com>'
 
 # Set root logger.
diff --git a/gaft/components/binary_individual.py b/gaft/components/binary_individual.py
index c9a68a1..b6b2274 100644
--- a/gaft/components/binary_individual.py
+++ b/gaft/components/binary_individual.py
@@ -14,21 +14,23 @@
 
 
 class BinaryIndividual(IndividualBase):
-    def __init__(self, ranges, eps=0.001):
-        '''
-        Class for individual in population. Random solution will be initialized
-        by default.
+    '''
+    Class for individual in population. Random solution will be initialized
+    by default.
 
-        NOTE: The decrete precisions for different components in varants may be
-              adjusted automatically (possible precision loss) if eps and ranges
-              are not appropriate.
+    :param ranges: value ranges for all entries in solution.
+    :type ranges: tuple list
 
-        :param ranges: value ranges for all entries in solution.
-        :type ranges: list of range tuples. e.g. [(0, 1), (-1, 1)]
+    :param eps: decrete precisions for binary encoding, default is 0.001.
+    :type eps: float or float list (with the same length with ranges)
 
-        :param eps: decrete precisions for binary encoding, default is 0.001.
-        :type eps: float or float list with the same length with ranges.
-        '''
+    .. Note:
+
+        The decrete precisions for different components in varants may be
+        adjusted automatically (possible precision loss) if eps and ranges
+        are not appropriate.
+    '''
+    def __init__(self, ranges, eps=0.001):
         super(self.__class__, self).__init__(ranges, eps)
 
         # Lengths for all binary sequence in chromsome and adjusted decrete precisions.
@@ -47,8 +49,7 @@ def __init__(self, ranges, eps=0.001):
         self.init()
 
     def encode(self):
-        '''
-        Encode solution to gene sequence in individual using different encoding.
+        ''' Encode solution to gene sequence in individual using different encoding.
         '''
         chromsome = []
         for var, (a, _), length, eps in zip(self.solution, self.ranges,
@@ -58,8 +59,7 @@ def encode(self):
         return chromsome
 
     def decode(self):
-        ''' 
-        Decode gene sequence to solution of target function.
+        ''' Decode gene sequence to solution of target function.
         '''
         solution =  [self.decimalize(self.chromsome[start: end], eps, lower_bound)
                      for (start, end), (lower_bound, _), eps in
@@ -76,12 +76,16 @@ def _get_gene_indices(self):
 
     @staticmethod
     def binarize(decimal, eps, length):
-        '''
-        Helper function to convert a float to binary sequence.
+        ''' Helper function to convert a float to binary sequence.
+
+        :param decimal: the decimal number to be converted
+        :type decimal: float
+
+        :param eps: the decrete precision of binary sequence
+        :type eps: float
 
-        :param decimal: the decimal number to be converted.
-        :param eps: the decrete precision of binary sequence.
         :param length: the length of binary sequence.
+        :type length: int
         '''
         n = int(decimal/eps)
         bin_str = '{:0>{}b}'.format(n, length)
@@ -89,8 +93,16 @@ def binarize(decimal, eps, length):
 
     @staticmethod
     def decimalize(binary, eps, lower_bound):
-        '''
-        Helper function to convert a binary sequence back to decimal number.
+        ''' Helper function to convert a binary sequence back to decimal number.
+
+        :param binary: The binary list to be converted
+        :type binary: list of int
+
+        :param eps: the decrete precision of binary sequence
+        :type eps: float
+
+        :param lower_bound: the lower bound for decimal number
+        :type lower_bound: float
         '''
         bin_str = ''.join([str(bit) for bit in binary])
         return lower_bound + int(bin_str, 2)*eps
diff --git a/gaft/components/decimal_individual.py b/gaft/components/decimal_individual.py
index 1d2791e..eb41975 100644
--- a/gaft/components/decimal_individual.py
+++ b/gaft/components/decimal_individual.py
@@ -8,6 +8,12 @@
 
 class DecimalIndividual(IndividualBase):
     ''' Individual with decimal encoding.
+
+    :param ranges: value ranges for all entries in solution.
+    :type ranges: tuple list
+
+    :param eps: decrete precisions for binary encoding, default is 0.001.
+    :type eps: float or float list (with the same length with ranges)
     '''
     def __init__(self, ranges, eps=0.001):
         super(self.__class__, self).__init__(ranges, eps)
@@ -15,8 +21,12 @@ def __init__(self, ranges, eps=0.001):
         self.init()
 
     def encode(self):
+        ''' Encode solution to gene sequence
+        '''
         return self.solution
 
     def decode(self):
+        ''' Decode gene sequence to decimal solution
+        '''
         return self.solution
 
diff --git a/gaft/components/individual.py b/gaft/components/individual.py
index eaa1ee5..0e59a6a 100644
--- a/gaft/components/individual.py
+++ b/gaft/components/individual.py
@@ -56,11 +56,17 @@ def __set__(self, obj, precisions):
 
 class IndividualBase(object):
     ''' Base class for individuals.
+
+    :param ranges: value ranges for all entries in solution.
+    :type ranges: tuple list
+
+    :param eps: decrete precisions for binary encoding, default is 0.001.
+    :type eps: float or float list (with the same length with ranges)
     '''
     # Solution ranges.
     ranges = SolutionRanges()
 
-    # Original decrete precisions (provided by users).
+    # Orginal decrete precisions (provided by users).
     eps = DecretePrecision()
     
     # Actual decrete precisions used in GA.
@@ -74,17 +80,17 @@ def __init__(self, ranges, eps):
         self.solution, self.chromsome = [], []
 
     def init(self, chromsome=None, solution=None):
-        '''
-        Initialize the individual by providing chromsome or solution.
-
-        If both chromsome and solution are provided, only the chromsome would
-        be used. If neither is provided, individual would be initialized randomly.
+        ''' Initialize the individual by providing chromsome or solution.
 
         :param chromsome: chromesome sequence for the individual
-        :type chromsome: list of float/int.
+        :type chromsome: list of (float / int)
 
         :param solution: the variable vector of the target function.
-        :type solution: list of float.
+        :type solution: list of float
+
+        .. Note::
+            If both chromsome and solution are provided, only the chromsome would
+            be used. If neither is provided, individual would be initialized randomly.
         '''
         if not any([chromsome, solution]):
             self.solution = self._rand_solution()
@@ -99,8 +105,7 @@ def init(self, chromsome=None, solution=None):
         return self
 
     def clone(self):
-        '''
-        Clone a new individual from current one.
+        ''' Clone a new individual from current one.
         '''
         indv = self.__class__(deepcopy(self.ranges), eps=deepcopy(self.eps))
         indv.init(chromsome=deepcopy(self.chromsome))
@@ -108,18 +113,22 @@ def clone(self):
 
 
     def encode(self):
-        ''' *NEED IMPLIMENTATION*
+        ''' **NEED IMPLIMENTATION**
+
         Convert solution to chromsome sequence.
 
-        :return chromsome: The chromsome sequence, float list.
+        :return: The chromsome sequence
+        :rtype: list of float
         '''
         raise NotImplementedError
 
     def decode(self):
-        ''' *NEED IMPLIMENTATION*
+        ''' **NEED IMPLIMENTATION**
+
         Convert chromsome sequence to solution.
 
-        :return solution: The solution vector, float list.
+        :return: The solution vector
+        :rtype: list of float
         '''
         raise NotImplementedError
 
diff --git a/gaft/components/population.py b/gaft/components/population.py
index 29130bd..8977cbe 100644
--- a/gaft/components/population.py
+++ b/gaft/components/population.py
@@ -5,8 +5,7 @@
 
 
 class Memoized(object):
-    '''
-    Descriptor for population statistical varibles caching.
+    ''' Descriptor for population statistical varibles caching.
     '''
     def __init__(self, func):
         self.func = func
@@ -34,8 +33,11 @@ def __call__(self, fitness):
 
 
 class Individuals(object):
-    '''
-    Descriptor for all individuals in population.
+    ''' Descriptor for all individuals in population.
+
+    .. Note::
+        Use this descriptor to ensure the individual related flags can be updated
+        when the population indivduals are changed.
     '''
     def __init__(self, name):
         self.name = '_{}'.format(name)
@@ -50,21 +52,19 @@ def __set__(self, instance, value):
 
 
 class Population(object):
+    ''' Class for representing population in genetic algorithm.
+
+    :param indv_template: A template individual to clone all the other
+                          individuals in current population.
+    :type indv_template: :obj:`gaft.components.IndividualBase`
 
+    :param size: The size of population, number of individuals in population.
+    :type size: int
+    '''
     # All individuals.
     individuals = Individuals('individuals')
 
     def __init__(self, indv_template, size=100):
-        '''
-        Class for representing population in genetic algorithm.
-
-        :param indv_template: A template individual to clone all the other
-                              individuals in current population.
-
-        :param size: The size of population, number of individuals in population.
-        :type size: int
-
-        '''
         # Population size.
         if size % 2 != 0:
             raise ValueError('Population size must be an even number')
@@ -78,8 +78,7 @@ def __init__(self, indv_template, size=100):
 
         # Container for all individuals.
         class IndvList(list):
-            '''
-            A proxy class inherited from built-in list to contain all
+            ''' A proxy class inherited from built-in list to contain all
             individuals which can update the population._updated flag
             automatically when its content is changed.
             '''
@@ -118,8 +117,7 @@ def extend(this, iterable_item):
         self._individuals = IndvList()
 
     def init(self, indvs=None):
-        '''
-        Initialize current population with individuals.
+        ''' Initialize current population with individuals.
 
         :param indvs: Initial individuals in population, randomly initialized
                       individuals are created if not provided.
@@ -146,21 +144,18 @@ def init(self, indvs=None):
         return self
 
     def update_flag(self):
-        '''
-        Interface for updating individual update flag to True.
+        ''' Interface for updating individual update flag to True.
         '''
         self._updated = True
 
     @property
     def updated(self):
-        '''
-        Query function for population updating flag.
+        ''' Query function for population updating flag.
         '''
         return self._updated
 
     def new(self):
-        '''
-        Create a new emtpy population.
+        ''' Create a new emtpy population.
         '''
         return self.__class__(indv_template=self.indv_template,
                               size=self.size)
@@ -180,45 +175,71 @@ def __len__(self):
         return len(self.individuals)
 
     def best_indv(self, fitness):
-        '''
-        The individual with the best fitness.
+        ''' The individual with the best fitness.
 
+        :param fitness: Fitness function to calculate fitness value
+        :type fitness: function
+
+        :return: the best individual in current population
+        :rtype: :obj:`gaft.components.IndividualBase`
         '''
         all_fits = self.all_fits(fitness)
         return max(self.individuals,
                    key=lambda indv: all_fits[self.individuals.index(indv)])
 
     def worst_indv(self, fitness):
-        '''
-        The individual with the worst fitness.
+        ''' The individual with the worst fitness.
+
+        :param fitness: Fitness function to calculate fitness value
+        :type fitness: function
+
+        :return: the worst individual in current population
+        :rtype: :obj:`gaft.components.IndividualBase`
         '''
         all_fits = self.all_fits(fitness)
         return min(self.individuals,
                    key=lambda indv: all_fits[self.individuals.index(indv)])
 
     def max(self, fitness):
-        '''
-        Get the maximum fitness value in population.
+        ''' Get the maximum fitness value in population.
+
+        :param fitness: Fitness function to calculate fitness value
+        :type fitness: function
+
+        :return: The maximum fitness value
+        :rtype: float
         '''
         return max(self.all_fits(fitness))
 
     def min(self, fitness):
-        '''
-        Get the minimum value of fitness in population.
+        ''' Get the minimum value of fitness in population.
+
+        :param fitness: Fitness function to calculate fitness value
+        :type fitness: function
+
+        :return: The minimum fitness value
+        :rtype: float
         '''
         return min(self.all_fits(fitness))
 
     def mean(self, fitness):
-        '''
-        Get the average fitness value in population.
+        ''' Get the average fitness value in population.
+
+        :param fitness: Fitness function to calculate fitness value
+        :type fitness: function
+
+        :return: The average fitness value
+        :rtype: float
         '''
         all_fits = self.all_fits(fitness)
         return sum(all_fits)/len(all_fits)
 
     @Memoized
     def all_fits(self, fitness):
-        '''
-        Get all fitness values in population.
+        ''' Get all fitness values in population.
+
+        :param fitness: Fitness function to calculate fitness value
+        :type fitness: function
         '''
         return [fitness(indv) for indv in self.individuals]
 
diff --git a/setup.py b/setup.py
index 963e534..ed5837f 100644
--- a/setup.py
+++ b/setup.py
@@ -31,7 +31,7 @@
     :target: https://www.python.org/downloads/release/python-351/
     :alt: platform
 
-.. image:: https://img.shields.io/badge/pypi-v0.6.0-blue.svg
+.. image:: https://img.shields.io/badge/pypi-v0.5.5-blue.svg
     :target: https://pypi.python.org/pypi/gaft/
     :alt: versions