diff --git a/scrapy/utils/reqser.py b/scrapy/utils/reqser.py
index 959dddbd5..8c99763cf 100644
--- a/scrapy/utils/reqser.py
+++ b/scrapy/utils/reqser.py
@@ -75,7 +75,11 @@ def _find_method(obj, func):
             pass
         else:
             if func_self is obj:
-                return six.get_method_function(func).__name__
+                name = six.get_method_function(func).__name__
+                if name.startswith('__'):
+                    classname = obj.__class__.__name__.lstrip('_')
+                    name = '_%s%s' % (classname, name)
+                return name
     raise ValueError("Function %s is not a method of: %s" % (func, obj))
 
 
diff --git a/tests/test_utils_reqser.py b/tests/test_utils_reqser.py
index dcc070b8f..f7191fcef 100644
--- a/tests/test_utils_reqser.py
+++ b/tests/test_utils_reqser.py
@@ -68,6 +68,12 @@ class RequestSerializationTest(unittest.TestCase):
                     errback=self.spider.handle_error)
         self._assert_serializes_ok(r, spider=self.spider)
 
+    def test_private_callback_serialization(self):
+        r = Request("http://www.example.com",
+                    callback=self.spider._TestSpider__parse_item_private,
+                    errback=self.spider.handle_error)
+        self._assert_serializes_ok(r, spider=self.spider)
+
     def test_unserializable_callback1(self):
         r = Request("http://www.example.com", callback=lambda x: x)
         self.assertRaises(ValueError, request_to_dict, r)
@@ -87,6 +93,9 @@ class TestSpider(Spider):
     def handle_error(self, failure):
         pass
 
+    def __parse_item_private(self, response):
+        pass
+
 
 class CustomRequest(Request):
     pass