Abstract: The optimal joint radius is proposed in the PMA driven joint system, which makes the output torque reach as maximum. Through analyzing the static characteristics of PMA, a static model of the joint is deduced. Based on simulations with different structure and state parameters of PMA, the relationship between the joint radius and the output torque is acquired. Simultaneously it is possible to calculate the optimal value of the joint radius. The greater joint angle is, the smaller optimal value of the joint radius is gained; the longer PMA is, the larger optimal value of the joint radius is gained. The diameter of a PMA has little effect on optimal value of the joint radius. This analysis method and results play an effective role in guiding the structural design of a dexterous hand driven by PMA.