Decoupled mechanisms are easy to control and apt to reach high accuracy. With the development of mechanism, especially the emergence of parallel mechanism, many ones don’t hold this merit. Especially, this merit is not an intrinsic property of parallel mechanisms. It's necessary to study their simple motions and name these motions decoupling motions. This kind of movement links very closely to motion control and pathway planning. By the use of screw theory, the definition, classification of decoupling motion, together with the necessary condition and several sufficient conditions for mechanism to realize this kind of motion are given. Finally a mechanism design example of a 2-DOF rotating mechanism is given as an example. It incarnates our research work on mechanism design and mechanism analysis.