Abstract: The evolution and mutation of planetary gear train with a small tooth differ-ence is discussed by means of kinematic inversion. Furthermore, a new mechanism having the same configuration with distinct properties is put forward.To explain the difference between the two mechanisms, the efficiency equations of the planetary gear train with a small tooth difference is derived first, followed with a study of the properties of the new mechansm,which is called an internal parralled translation gear train.The new mechanism is proved to be superior to the planetary gear with a small tooth difference.