diff --git a/onnxruntime/python/tools/quantization/onnx_model.py b/onnxruntime/python/tools/quantization/onnx_model.py
index 0cda0a4a59..8e6d70c4bb 100644
--- a/onnxruntime/python/tools/quantization/onnx_model.py
+++ b/onnxruntime/python/tools/quantization/onnx_model.py
@@ -260,38 +260,49 @@ class ONNXModel:
     def topological_sort(self):
         deps_count = [0]*len(self.nodes()) # dependency count of each node
         deps_to_nodes = {} # input to node indice
+        sorted_nodes = []  # initialize sorted_nodes
         for node_idx, node in enumerate(self.nodes()):
             # CANNOT use len(node.input) directly because input can be optional
             deps_count[node_idx] = sum(1 for _ in node.input if _ )
+            if deps_count[node_idx] == 0: # Constant doesn't depend on any inputs
+                sorted_nodes.append(self.nodes()[node_idx])
+                continue
+
             for input_name in node.input:
                 if input_name not in deps_to_nodes:
                     deps_to_nodes[input_name] = [node_idx]
                 else:
                     deps_to_nodes[input_name].append(node_idx)
 
-        # initialize sorted_nodes
-        sorted_nodes = []
-        for input in itertools.chain(self.initializer(), self.model.graph.input):
-            if input.name in deps_to_nodes:
-                for node_idx in deps_to_nodes[input.name]:
+        initializer_names = [init.name for init in self.initializer()]
+        graph_input_names = [input.name for input in self.model.graph.input]
+        input_names = initializer_names + graph_input_names
+        input_names.sort()
+        prev_input_name = None
+        for input_name in input_names:
+            if prev_input_name == input_name:
+                continue
+
+            prev_input_name = input_name
+            if input_name in deps_to_nodes:
+                for node_idx in deps_to_nodes[input_name]:
                     deps_count[node_idx] = deps_count[node_idx] - 1
                     if deps_count[node_idx] == 0:
                         sorted_nodes.append(self.nodes()[node_idx])
 
-        s = 0
-        e = len(sorted_nodes)
+        start = 0
+        end = len(sorted_nodes)
 
-        while s < e:
-            for output in sorted_nodes[s].output:
+        while start < end:
+            for output in sorted_nodes[start].output:
                 if output in deps_to_nodes:
                     for node_idx in deps_to_nodes[output]:
                         deps_count[node_idx] = deps_count[node_idx] - 1
                         if deps_count[node_idx] == 0:
                             sorted_nodes.append(self.nodes()[node_idx])
-                            e = e + 1
-            s = s + 1
+                            end = end + 1
+            start = start + 1
 
-        assert(e == len(self.graph().node)), "Graph is not a DAG"
+        assert(end == len(self.graph().node)), "Graph is not a DAG"
         self.graph().ClearField('node')
-        self.graph().node.extend(sorted_nodes)
-
+        self.graph().node.extend(sorted_nodes)
\ No newline at end of file
diff --git a/onnxruntime/test/python/quantization/test_onnx_model.py b/onnxruntime/test/python/quantization/test_onnx_model.py
index 7d98b53b2e..b1d1736639 100644
--- a/onnxruntime/test/python/quantization/test_onnx_model.py
+++ b/onnxruntime/test/python/quantization/test_onnx_model.py
@@ -65,6 +65,28 @@ class TestONNXModel(unittest.TestCase):
         model = helper.make_model(graph, opset_imports=[helper.make_opsetid("", 13)])
         onnx.save(model, model_path)
 
+    def construct_model_Constant(self, model_path):
+        #    (input)    Constant
+        #       \         /
+        #        \       /
+        #         \     /
+        #          \   /
+        #           Add
+        #            |
+        #         (output)
+
+        initializers = []
+        input = helper.make_tensor_value_info('input', TensorProto.FLOAT, [4, 8, 12])
+        output = helper.make_tensor_value_info('output', TensorProto.FLOAT, [4, 8, 12])
+
+        # make nodes
+        constant_node = onnx.helper.make_node('Constant', [], ['const_output'], value_float=42.0)
+        add_node = onnx.helper.make_node('Add', ['input', 'const_output'], ['output'], name='Add')
+        graph = helper.make_graph([add_node, constant_node],
+                                  'onnx_model_test', [input], [output], initializer=initializers)
+        model = helper.make_model(graph, opset_imports=[helper.make_opsetid("", 13)])
+        onnx.save(model, model_path)
+
     def test_topo_sort(self):
         test_model_path = 'onnx_model_topo_sort.onnx'
         self.construct_model(test_model_path)
@@ -73,5 +95,13 @@ class TestONNXModel(unittest.TestCase):
         onnx_model.topological_sort()
         check_op_type_order(self, onnx_model.model, ['GRU', 'Conv', 'Conv', 'Relu', 'Add'])
 
+    def test_topo_sort_constant(self):
+        test_model_path = 'onnx_model_topo_sort_constant.onnx'
+        self.construct_model_Constant(test_model_path)
+        onnx_model = ONNXModel(onnx.load(test_model_path))
+        check_op_type_order(self, onnx_model.model, ['Add', 'Constant'])
+        onnx_model.topological_sort()
+        check_op_type_order(self, onnx_model.model, ['Constant', 'Add'])
+
 if __name__ == '__main__':
     unittest.main()