diff --git a/diagrams/__init__.py b/diagrams/__init__.py
index 1fa343b0..eef4edbf 100644
--- a/diagrams/__init__.py
+++ b/diagrams/__init__.py
@@ -196,11 +196,11 @@ class Diagram:
 
         if not self._validate_direction(direction):
             raise ValueError(f'"{direction}" is not a valid direction')
-        self.dot.graph_attr["rankdir"] = direction
+        self.dot.graph_attr["rankdir"] = direction.upper()
 
         if not self._validate_curvestyle(curvestyle):
             raise ValueError(f'"{curvestyle}" is not a valid curvestyle')
-        self.dot.graph_attr["splines"] = curvestyle
+        self.dot.graph_attr["splines"] = curvestyle.lower()
 
         if isinstance(outformat, list):
             for one_format in outformat:
diff --git a/tests/test_diagram.py b/tests/test_diagram.py
index 75fc02b8..32564555 100644
--- a/tests/test_diagram.py
+++ b/tests/test_diagram.py
@@ -43,6 +43,11 @@ class DiagramTest(unittest.TestCase):
             with self.assertRaises(ValueError):
                 Diagram(curvestyle=cvs)
 
+    def test_case_insensitive_graph_attributes(self):
+        diagram = Diagram(direction="lr", curvestyle="CURVED")
+        self.assertEqual(diagram.dot.graph_attr["rankdir"], "LR")
+        self.assertEqual(diagram.dot.graph_attr["splines"], "curved")
+
     def test_validate_theme(self):
         # Valid themes.
         for theme in ("neutral", "pastel", "blues", "greens", "orange"):