On the assumption of FCWF model, the conditions of three-fingered robot hand force-closure grasping are equivalent to grasp matrix be full rank and exist rigorous internal forces. Based on the analysis to the geometrical characteristics of contact points, the existence condition of the frictional sectors is established. By means of studying the relative location relationships between the frictional sector borderlines and their intersections, the existence condition of the concurrent polygon of three internal forces is presented. Taking the equilibrium relation of internal forces into consideration, a general algorithm using vector calculation for force-closure grasping is established. An example is given to illustrate the validity of the algorithm.