diff --git a/onnxscript/_internal/_inliner.py b/onnxscript/_internal/_inliner.py
index 6a4d6d6742..ba38f573e5 100644
--- a/onnxscript/_internal/_inliner.py
+++ b/onnxscript/_internal/_inliner.py
@@ -10,29 +10,29 @@
 
 
 def instantiate(
-    function: ir.Function,
+    graph: ir.Graph,
     inputs: Sequence[ir.Value | None],
     attributes: Mapping[str, ir.Attr],
     *,
     prefix: str = "",
 ) -> tuple[list[ir.Node], list[ir.Value | None]]:
-    """Instantiate (inline) a function, substituting inputs and attributes.
+    """Instantiate (inline) a graph, substituting inputs and attributes.
 
     Args:
-        function: The function to instantiate.
-        inputs: Actual input values to bind to the function's formal parameters.
+        graph: The graph to instantiate.
+        inputs: Actual input values to bind to the graph's formal parameters.
         attributes: Attribute values to substitute for reference attributes.
         prefix: Optional prefix to prepend to node and output names.
 
     Returns:
-        A tuple of (nodes, outputs) where nodes are the cloned function body
-        and outputs are the values corresponding to the function's outputs.
+        A tuple of (nodes, outputs) where nodes are the cloned graph body
+        and outputs are the values corresponding to the graph's outputs.
     """
-    formal_inputs = function.inputs
+    formal_inputs = graph.inputs
     if len(inputs) > len(formal_inputs):
         raise ValueError(
             f"Too many inputs: got {len(inputs)}, "
-            f"but function has {len(formal_inputs)} parameters."
+            f"but graph has {len(formal_inputs)} parameters."
         )
     value_map: dict[ir.Value, ir.Value | None] = dict(zip(formal_inputs, inputs))
 
@@ -50,7 +50,8 @@ def rename(node: ir.Node) -> None:
         metadata_props={},
         post_process=rename,
         resolve_ref_attrs=True,
+        allow_outer_scope_values=True,
     )
-    nodes = [cloner.clone_node(n) for n in function]
-    outputs = [value_map.get(v) for v in function.outputs]
+    nodes = [cloner.clone_node(n) for n in graph]
+    outputs = [value_map.get(v) for v in graph.outputs]
     return nodes, outputs
diff --git a/onnxscript/_internal/builder.py b/onnxscript/_internal/builder.py
index fb81f3451f..06b6edaa85 100644
--- a/onnxscript/_internal/builder.py
+++ b/onnxscript/_internal/builder.py
@@ -450,25 +450,24 @@ def call(
         **kwargs,
     ):
         if isinstance(function, ir.Function):
-            function_ir = function
+            graph = function.graph
         elif isinstance(function, onnxscript.OnnxFunction):
-            function_proto = function.to_function_proto()
-            function_ir = ir.serde.deserialize_function(function_proto)
+            graph = function.graph()
         else:
             raise TypeError("Function must be an ir.Function or onnxscript.OnnxFunction")
         output_renaming: dict[str, str] = {}
         if _outputs is not None:
-            if len(_outputs) != len(function_ir.outputs):
+            if len(_outputs) != len(graph.outputs):
                 raise ValueError(
                     f"Number of provided output names {_outputs} does not match "
-                    f"number of function outputs {len(function_ir.outputs)}."
+                    f"number of function outputs {len(graph.outputs)}."
                 )
-            for output, name in zip(function_ir.outputs, _outputs):
+            for output, name in zip(graph.outputs, _outputs):
                 output_renaming[output.name] = self._qualify_value_name(name)
         else:
-            for output in function_ir.outputs:
+            for output in graph.outputs:
                 output_renaming[output.name] = self._qualify_value_name(output.name)
-        nodes, outputs = _inliner.instantiate(function_ir, args, kwargs)
+        nodes, outputs = _inliner.instantiate(graph, args, kwargs)
         if _prefix:
             self.push_module(_prefix)
         for node in nodes:
diff --git a/onnxscript/_internal/builder_test.py b/onnxscript/_internal/builder_test.py
index 1ff8460b67..ffc1ab44a4 100644
--- a/onnxscript/_internal/builder_test.py
+++ b/onnxscript/_internal/builder_test.py
@@ -10,6 +10,7 @@
 import onnx_ir as ir
 
 import onnxscript._internal.builder as builder
+import onnxscript.testing
 from onnxscript import script
 from onnxscript.onnx_types import DOUBLE, FLOAT
 
@@ -713,6 +714,31 @@ def add_mul(X, Y):
         self.assertEqual(nodes[0].op_type, "Add")
         self.assertEqual(nodes[1].op_type, "Mul")
 
+    def test_call_with_outer_scope_value(self):
+        """Test that script supports references to pre-existing values."""
+        # Create a GraphBuilder first
+        op, x, y = _create_builder_with_inputs()
+        product = op.Mul(x, y)
+
+        @script()
+        def add_product(X):
+            return op.Add(X, product)  # Reference to 'product' from outer scope
+
+        x_plus = op.call(add_product, x, _outputs=["x_plus"])
+        y_plus = op.call(add_product, y, _outputs=["y_plus"])
+
+        op.builder.graph.outputs.extend([x_plus, y_plus])
+
+        # Now, create the same graph directly:
+        op2, x2, y2 = _create_builder_with_inputs()
+        product2 = op2.Mul(x2, y2)
+        x2_plus = op2.Add(x2, product2, _outputs=["x_plus"])
+        y2_plus = op2.Add(y2, product2, _outputs=["y_plus"])
+        op2.builder.graph.outputs.extend([x2_plus, y2_plus])
+
+        # Verify that the two graphs are structurally equivalent
+        onnxscript.testing.assert_isomorphic_graph(op.builder.graph, op2.builder.graph)
+
     def test_call_with_prefix_option(self):
         """Test that GraphBuilder.call respects the _prefix option for hierarchical naming."""
         # Create a GraphBuilder first
diff --git a/onnxscript/_internal/converter.py b/onnxscript/_internal/converter.py
index dd215a7c06..468aa41675 100644
--- a/onnxscript/_internal/converter.py
+++ b/onnxscript/_internal/converter.py
@@ -377,6 +377,9 @@ def _to_onnx_var(
         if isinstance(val, values.SymbolValue):
             if isinstance(val.value, ir.Value):
                 return val.value
+        if isinstance(val, ir.Value):
+            # An outer-scope ir.Value (e.g., from a closure variable) can be used directly.
+            return val
         # Assume value is a python-value convertible to a tensor
         # TODO: check if value is convertible to a TensorProto, so that we can
         # produce a better error _message otherwise
diff --git a/onnxscript/_internal/values.py b/onnxscript/_internal/values.py
index 0019802f36..051cb3e686 100644
--- a/onnxscript/_internal/values.py
+++ b/onnxscript/_internal/values.py
@@ -333,6 +333,15 @@ def to_function_proto(self) -> onnx.FunctionProto:
         """Converts the function into :class:`onnx.FunctionProto`."""
         return self.function_ir.to_function_proto()
 
+    def graph(self) -> ir.Graph:
+        """Returns the IR graph representation of this function.
+
+        Returns:
+            The :class:`ir.Graph` representing the computation graph of this function.
+            NOTE: This is not a copy, and should not be modified by the caller.
+        """
+        return self.function_ir.graph
+
     def to_model_proto(self, **kwargs):
         """Converts the function into :class:`onnx.ModelProto`."""
         if self.function_ir.attrs and any(
diff --git a/onnxscript/testing/__init__.py b/onnxscript/testing/__init__.py
index 048b45e7e8..0b40a4aa35 100644
--- a/onnxscript/testing/__init__.py
+++ b/onnxscript/testing/__init__.py
@@ -320,6 +320,8 @@ def _to_function_proto(f):
         return f.to_function_proto()
     if isinstance(f, str):
         return parser.parse_function(f)
+    if isinstance(f, ir.Function):
+        return ir.to_proto(f)
     raise TypeError(f"Cannot convert {type(f)} to FunctionProto")
 
 
@@ -330,6 +332,8 @@ def _to_graph_proto(g):
         return g.to_model_proto().graph
     if isinstance(g, str):
         return parser.parse_graph(g)
+    if isinstance(g, ir.Graph):
+        return ir.to_proto(g)
     raise TypeError(f"Cannot convert {type(g)} to ModelProto")
 
 
@@ -342,6 +346,10 @@ def _to_function_or_graph(obj):
         return obj.graph
     if isinstance(obj, onnxscript.OnnxFunction):
         return obj.to_function_proto()
+    if isinstance(obj, ir.Function):
+        return ir.to_proto(obj)
+    if isinstance(obj, ir.Graph):
+        return ir.to_proto(obj)
     raise TypeError(f"Cannot convert {type(obj)} to FunctionProto or GraphProto")