diff --git a/tests/core/test_unflatten.py b/tests/core/test_unflatten.py
index 4334942..d1931ec 100644
--- a/tests/core/test_unflatten.py
+++ b/tests/core/test_unflatten.py
@@ -8,5 +8,71 @@ import unittest
 class unflatten_test_case(unittest.TestCase):
 
     def test_unflatten(self):
-        # TODO
-        pass
+        d = {
+            'a': 1,
+            'b_c': 2,
+            'd_e': 3,
+        }
+        u = _unflatten(d)
+        r = {
+            'a': 1,
+            'b': {
+                'c': 2,
+            },
+            'd': {
+                'e': 3,
+            },
+        }
+        self.assertEqual(u, r)
+
+    def test_unflatten_with_custom_separator(self):
+        d = {
+            'a': 1,
+            'b|c': 2,
+            'd|e': 3,
+        }
+        u = _unflatten(d, separator='#')
+        self.assertEqual(u, d)
+        u = _unflatten(d, separator='|')
+        r = {
+            'a': 1,
+            'b': {
+                'c': 2,
+            },
+            'd': {
+                'e': 3,
+            },
+        }
+        self.assertEqual(u, r)
+
+    def test_unflatten_with_nested_dict(self):
+        d = {
+            'a': 1,
+            'b_c': {
+                'u_v': 2,
+            },
+            'd_e': {
+                'x_y_z': 3,
+            },
+        }
+        u = _unflatten(d)
+        r = {
+            'a': 1,
+            'b': {
+                'c': {
+                    'u': {
+                        'v': 2,
+                    },
+                },
+            },
+            'd': {
+                'e': {
+                    'x': {
+                        'y': {
+                            'z': 3,
+                        },
+                    },
+                },
+            },
+        }
+        self.assertEqual(u, r)