Abstract: According to the principle of direct compliance control,this paper discusses the ralation betweenthe stiffness of the end-effector and the pose of compliance mechanism for a parallel link arm.In this paperthe stiffness ratio is quoted for a index of adjustable performance of end-effector'stiffness.Available valueof the stiffness ratio is limited.Controllability of end-effector's stiffness is best with K_x/K_y=1,and operat-ing range of end-effector is ralated to the stiffness ratio.