diff --git a/Assets/MGS-CommonCode/Machinery/Mechanism.cs b/Assets/MGS-CommonCode/Machinery/Mechanism.cs
index be80ffe..5da905e 100644
--- a/Assets/MGS-CommonCode/Machinery/Mechanism.cs
+++ b/Assets/MGS-CommonCode/Machinery/Mechanism.cs
@@ -8,6 +8,11 @@
  *  Version      :  0.1.0
  *  Date         :  4/17/2018
  *  Description  :  Initial development version.
+ *  
+ *  Author       :  Mogoson
+ *  Version      :  0.1.0
+ *  Date         :  5/24/2018
+ *  Description  :  Define mechanism interface.
  *************************************************************************/
 
 using System;
@@ -16,41 +21,21 @@
 namespace Mogoson.Machinery
 {
     /// <summary>
-    /// Mechanism unit.
+    /// Mechanism interface.
     /// </summary>
-    [Serializable]
-    public struct MechanismUnit
+    public interface IMechanism
     {
-        #region Field and Property
-        /// <summary>
-        /// Mechanism to drive.
-        /// </summary>
-        public Mechanism mechanism;
-
         /// <summary>
-        /// Coefficient of velocity.
-        /// </summary>
-        public float coefficient;
-        #endregion
-
-        #region Public Method
-        /// <summary>
-        /// Constructor.
+        /// Drive mechanism.
         /// </summary>
-        /// <param name="mechanism">Mechanism to drive.</param>
-        /// <param name="coefficient">Coefficient of velocity.</param>
-        public MechanismUnit(Mechanism mechanism, float coefficient)
-        {
-            this.mechanism = mechanism;
-            this.coefficient = coefficient;
-        }
-        #endregion
+        /// <param name="velocity">Drive velocity.</param>
+        void Drive(float velocity);
     }
 
     /// <summary>
     /// Base mechanism.
     /// </summary>
-    public abstract class Mechanism : MonoBehaviour
+    public abstract class Mechanism : MonoBehaviour, IMechanism
     {
         #region Protected Method
         protected virtual void Awake()
@@ -72,4 +57,36 @@ public virtual void Initialize() { }
         public abstract void Drive(float velocity);
         #endregion
     }
+
+    /// <summary>
+    /// Mechanism unit.
+    /// </summary>
+    [Serializable]
+    public struct MechanismUnit
+    {
+        #region Field and Property
+        /// <summary>
+        /// Mechanism to drive.
+        /// </summary>
+        public Mechanism mechanism;
+
+        /// <summary>
+        /// Coefficient of velocity.
+        /// </summary>
+        public float coefficient;
+        #endregion
+
+        #region Public Method
+        /// <summary>
+        /// Constructor.
+        /// </summary>
+        /// <param name="mechanism">Mechanism to drive.</param>
+        /// <param name="coefficient">Coefficient of velocity.</param>
+        public MechanismUnit(Mechanism mechanism, float coefficient)
+        {
+            this.mechanism = mechanism;
+            this.coefficient = coefficient;
+        }
+        #endregion
+    }
 }
\ No newline at end of file
diff --git a/Assets/MGS-CommonCode/Machinery/Transmission.cs b/Assets/MGS-CommonCode/Machinery/Transmission.cs
index 4ff5354..724584c 100644
--- a/Assets/MGS-CommonCode/Machinery/Transmission.cs
+++ b/Assets/MGS-CommonCode/Machinery/Transmission.cs
@@ -23,7 +23,7 @@ public class Transmission : Mechanism
     {
         #region Field and Property
         /// <summary>
-        /// Mechanism drive by this Transmission.
+        /// Mechanism drive by this transmission.
         /// </summary>
         public List<MechanismUnit> mechanismUnits = new List<MechanismUnit>();
         #endregion