diff --git a/brainpy/math/oo_transform.py b/brainpy/math/oo_transform.py
index 548a987d..7654731d 100644
--- a/brainpy/math/oo_transform.py
+++ b/brainpy/math/oo_transform.py
@@ -59,3 +59,7 @@
   eval_shape as eval_shape,
 )
 
+from brainpy._src.math.object_transform.variables import (
+  VariableStack as VariableStack,
+)
+
diff --git a/docs/apis/brainpy.math.oo_transform.rst b/docs/apis/brainpy.math.oo_transform.rst
index 754e0d81..9ed9cf46 100644
--- a/docs/apis/brainpy.math.oo_transform.rst
+++ b/docs/apis/brainpy.math.oo_transform.rst
@@ -77,4 +77,5 @@ Helpers for Object-oriented Transformations
    :template: classtemplate.rst
 
    eval_shape
+   VariableStack