Abstract: A design method for manipulators having simple dynamics is presented. First,one or more linkgroups which have zero DOF are added to an open-loop chain to compose a closed-loop chain. Then the massdistribution of the link groups is designed in order to decouple the dynamics of the closed-loop linkage. As aresult,we can design a planar 3 DOF arm with decoupled and constant inertia matrix. Because of its simpleinertia matrix,the arm has good promise for high speed manipulation.