Power grasp is redefined as a type of grasp that its mechanism can resist passively against external forces without relying on feedback control of joint torques. A computational algorithm is invented to calculate the critical external force, a force which is requisite to move the grasped object in a definite direction. Virtual work is proposed as the quality measure of the robustness of power grasp. Because this measure is a scalar, it is convenient and suitable for the planning of power grasp. The effectiveness of the computational algorithm of critical external force and quality measure of robustness of power grasp is verified with a numerical example.>