Abstarct: A correct understanding of the mechanical properties (deformability and strength) of jointed rock masses is essential for analyzing, designing and performance estimation of structures built in and on rock masses. Due to the existence of various discontinuities and the inherent statistical nature of their geometrical and geomechanical properties, it is very difficult to predict accurately the strength and deformability properties of jointed rock masses. In this research, Firstly, baxsed on engineering judgment and experience, there is a basic division of rocks into three categories: igneous, mextamorphic and sedimentary rock is made. Then, by field joint survey from all three types of rocks: igneous, sedimentary and mextamorphic, distribution function of joint trace length are estimated. Then by using the joint trace length distribution function, the statistical model was constructed for numerical analysis. After compiling the statistical model, using numerical methods, “discrete element method” (UDEC software) as a tool for stress analysis in two dimensions on blocks of jointed rock mass with different joints, joint trace length factor and its effect on the ultimate strength of jointed rock masses has been studied.