Abstract: The problem presented in this paper is a stability analysis of rock block, separated by a set of two geologic structural surfaces from slope rockmass and loaded by a 3 D force system without common point of force action. The method developed in this paper is based upon the use of vector analysis, the principle of nodal equivalent forces, and that the distribution of friction force at any point on a geologic structural surface is treated as a function of the normal pressure acted on that point on the surface and the relative displacement of the block at that point along the surface. Thus the block movement may involve not only parallel translation, but also sliding rotation and other 3 D movement. Finally, a computer program is proposed to solve the stability of rock block for various possible modes of its instability.