diff --git a/python/paddle/v2/parameters.py b/python/paddle/v2/parameters.py
index d0b5ff12f2748909745294d4ba96888f2baf2b8d..bd97dc1199fedc8ac91c1c6086957e8cce88bdc4 100644
--- a/python/paddle/v2/parameters.py
+++ b/python/paddle/v2/parameters.py
@@ -326,6 +326,17 @@ class Parameters(object):
         self.set(name, arr.reshape(self.get_shape(name)))
 
     def to_tar(self, f):
+        """
+        Save parameters to a tar file.
+
+        WARNING: You should use `paddle.v2.trainer.SGD.save_parameter_to_tar(f)`
+            to save parameters most of the time. Otherwise, some settings such
+            as model average will not take effect.
+
+        :param f:
+        :type f: file
+        :return:
+        """
         tar = tarfile.TarFile(fileobj=f, mode='w')
         for nm in self.names():
             buf = cStringIO.StringIO()