diff --git a/onnxscript/main.py b/onnxscript/main.py
index bfcbf0bc4b..7407baedd1 100644
--- a/onnxscript/main.py
+++ b/onnxscript/main.py
@@ -6,14 +6,18 @@
 import ast
 import inspect
 import sys
-from typing import Any, Callable, Optional, Sequence
+from typing import Any, Callable, Optional, Sequence, TypeVar
 
 import onnx.helper
+from typing_extensions import ParamSpec
 
 import onnxscript
 from onnxscript import converter, irbuilder, values
 from onnxscript._internal import ast_utils
 
+_R = TypeVar("_R")
+_P = ParamSpec("_P")
+
 
 def script_check(
     f: ast.FunctionDef,
@@ -39,7 +43,7 @@ def script(
     opset: Optional[values.Opset] = None,
     default_opset: Optional[values.Opset] = None,
     **kwargs: Any,
-) -> Callable[[Callable], onnxscript.OnnxFunction]:
+) -> Callable[[Callable[_P, _R]], onnxscript.OnnxFunction[_P, _R]]:
     """Main decorator. Declares a function as an onnx function.
 
     Args:
@@ -75,7 +79,7 @@ def log2(x):
             "Script parameter must be an opset. Did you use @script instead of @script()?"
         )
 
-    def transform(f: Callable) -> onnxscript.OnnxFunction:
+    def transform(f: Callable[_P, _R]) -> onnxscript.OnnxFunction[_P, _R]:
         if not inspect.isfunction(f):
             raise TypeError("The ONNXScript decorator should be applied to functions only.")
 
diff --git a/onnxscript/values.py b/onnxscript/values.py
index 9907b16ee4..d748dc6e64 100644
--- a/onnxscript/values.py
+++ b/onnxscript/values.py
@@ -13,20 +13,27 @@
     Any,
     Callable,
     ClassVar,
+    Generic,
     Optional,
     Protocol,
     Sequence,
+    TypeVar,
     _GenericAlias,
 )
 
 import onnx
 import onnx.defs
+from typing_extensions import ParamSpec
 
 from onnxscript import converter as converter_module
 from onnxscript import irbuilder, sourceinfo, type_annotation
 from onnxscript._internal import ast_utils, deprecation
 from onnxscript.ir import _schemas
 
+_R = TypeVar("_R")
+_P = ParamSpec("_P")
+
+
 _ATTRIBUTE_TYPE_TO_PYTHON_TYPE = {
     onnx.defs.OpSchema.AttrType.FLOAT: float,
     onnx.defs.OpSchema.AttrType.INT: int,
@@ -464,7 +471,7 @@ def _op_schema_from_function_ir(
     )
 
 
-class OnnxFunction(Op):
+class OnnxFunction(Op, Generic[_P, _R]):
     """Represents an ONNX op for which a function-body has been defined in onnxscript.
 
     Attributes:
@@ -566,12 +573,12 @@ def fun(*args, **kwargs):
 
         return fun
 
-    def __call__(self, *args, **kwargs):
+    def __call__(self, *args: _P.args, **kwargs: _P.kwargs) -> _R:
         """Implements an eager-mode execution of an onnxscript function."""
         # FIXME(after #225): Move import to the top of the file.
         from onnxscript import evaluator  # pylint: disable=import-outside-toplevel
 
-        return evaluator.default().eval_function(self, args, kwargs)
+        return evaluator.default().eval_function(self, args, kwargs)  # type: ignore[arg-type, return-value]
 
     def __repr__(self) -> str:
         return f"{self.__class__.__name__}({self.function!r})"