diff --git a/ZeroLevel.UnitTests/LCSTest.cs b/ZeroLevel.UnitTests/LCSTest.cs
new file mode 100644
index 0000000..1471e03
--- /dev/null
+++ b/ZeroLevel.UnitTests/LCSTest.cs
@@ -0,0 +1,47 @@
+using Xunit;
+using ZeroLevel.Semantic;
+
+namespace ZeroLevel.UnitTests
+{
+    public class LCSTest
+    {
+        [Fact]
+        public void LCSTest1()
+        {
+            // Arrange
+            var s_base = "abcdefghijklmnopqrstuvwxyz";
+            var s1 = "klmnO";
+
+            var s2 = "";
+            string s3 = null;
+
+            // Act
+            var st11 = LongestCommonSubstring.LCS(s_base, s1);
+            var st12 = LongestCommonSubstring.LCS(s1, s_base);
+
+            var st13 = LongestCommonSubstring.LCSIgnoreCase(s_base, s1);
+            var st14 = LongestCommonSubstring.LCSIgnoreCase(s1, s_base);
+
+            var st21 = LongestCommonSubstring.LCS(s_base, s2);
+            var st22 = LongestCommonSubstring.LCS(s2, s_base);
+
+            var st31 = LongestCommonSubstring.LCS(s_base, s3);
+            var st32 = LongestCommonSubstring.LCS(s3, s_base);
+
+            //Assert
+            Assert.Equal("klmn", st11);
+            Assert.Equal("klmn", st12);
+
+            Assert.Equal(s1, st13, ignoreCase: true);
+            Assert.Equal(s1, st14, ignoreCase: true);
+
+
+            Assert.Equal(string.Empty, st21);
+            Assert.Equal(string.Empty, st22);
+                         
+            Assert.Equal(string.Empty, st31);
+            Assert.Equal(string.Empty, st32);
+
+        }
+    }
+}
diff --git a/ZeroLevel/Services/Semantic/LongestCommonSubstring.cs b/ZeroLevel/Services/Semantic/LongestCommonSubstring.cs
new file mode 100644
index 0000000..1b3984f
--- /dev/null
+++ b/ZeroLevel/Services/Semantic/LongestCommonSubstring.cs
@@ -0,0 +1,47 @@
+namespace ZeroLevel.Semantic
+{
+    public static class LongestCommonSubstring
+    {
+        public static string LCS(string s1, string s2)
+        {
+            if (s1 == null || s2 == null || s1.Length == 0 || s2.Length == 0) return string.Empty;
+            var a = new int[s1.Length + 1, s2.Length + 1];
+            int u = 0, v = 0;
+
+            for (var i = 0; i < s1.Length; i++)
+                for (var j = 0; j < s2.Length; j++)
+                    if (s1[i] == s2[j])
+                    {
+                        a[i + 1, j + 1] = a[i, j] + 1;
+                        if (a[i + 1, j + 1] > a[u, v])
+                        {
+                            u = i + 1;
+                            v = j + 1;
+                        }
+                    }
+
+            return s1.Substring(u - a[u, v], a[u, v]);
+        }
+
+        public static string LCSIgnoreCase(string s1, string s2)
+        {
+            if (s1 == null || s2 == null || s1.Length == 0 || s2.Length == 0) return string.Empty;
+            var a = new int[s1.Length + 1, s2.Length + 1];
+            int u = 0, v = 0;
+
+            for (var i = 0; i < s1.Length; i++)
+                for (var j = 0; j < s2.Length; j++)
+                    if (char.ToUpperInvariant(s1[i]) == char.ToUpperInvariant(s2[j]))
+                    {
+                        a[i + 1, j + 1] = a[i, j] + 1;
+                        if (a[i + 1, j + 1] > a[u, v])
+                        {
+                            u = i + 1;
+                            v = j + 1;
+                        }
+                    }
+
+            return s1.Substring(u - a[u, v], a[u, v]);
+        }
+    }
+}