Abstract: On the basis that both series mechanisms and parallel mechanisms have only one degree of freedom, and based on the theory of mechanism composition, the composition between a mechanism having one degree of freedom and a mechanism having multi degree of freedom is defined as a closed composition. Examples of composition involving a mechanism having one degree of freedom and anether mechanism having two degrees of freedom proved the above concept. It is clear and easy to understand mechanism composition and to create a new mechanism system with this concept of closed composition.