When an object is grasped by a multifingered hand with power grasp, there are an infinite number of possible power grasp forms, provided by combinations of the hand shape, the contact positions between fingers and the object, and the joint torques of the hand. To plan a power grasp, it is necessary to know, by some prior evaluation which form is most suitable among these forms. This paper proposes a technique for automatically obtaining optimal power grasp. By analyzing the degree of freedom of the force space caused by frictional contacts between fingers and object, the conditions and mechanical properties of power grasp are described. Also, the region of feasible joint torques for power grasp is derived. Then, an optimal power grasp form is defined and its determination procedure is established.