Abstarct: A rock mass consists of intact rock and various forms of discontinuity. Fluid flow through rock mass, transfers in the fractures and porous medium. In hard rock, the discrete approach is favoured in which the flow is assumed to be mainly through an interconnected fracture network. Fracture geometry includes Opening, roughness, orientation and…, is the determinative factor inthe fluid flow rate through the fractures. In this research work, the effect ofjoint surface roughness has been evaluated. In different studies, joint surface roughness considered to be as the coefficient cubic law, changing the opening exponent of the cubic law or change the value of opening of the joint. In this dissertation for study the effect of joint roughness surface (JRC) Barton-Bandis joint model has been used which is computed to the effect of roughness as the reduce of opening of fracture. In this regard, in first step, the effect of roughness on flow rate through single joint has been studied and results have been compared with the parallel plate model and it also is assessed with the results of experimental studies. In the following,, numerical analysis has been investigated for evaluation the effect of surface roughness on the flow rate through the joints in both two-dimensional and three-dimensional cases. Also, two-dimensional and three-dimensional actual models of rock masses, situated in the Sanandaj-Hamadan road, have been created using the concept of discrete fracture network (DFN), in the 3DEC software. The flow rate through two-dimensional DFN models have been calculated for three cases including smooth joints, an average roughness joints and an variable roughness joints uding UDEC software and then they are compared together in thequality and quantity point of view. Fortheremore flow rates in both mechanical and hydraulic opening modeling cases have been modeled for three-dimensional model, by using of 3DEC software. The results show that the effect of surface roughness is significant on the fluid flow rate through the joints and the joint surface roughness, caused the changes in velocity and flow rate. Since in cubic law the exponent of opening is 3, so it is considered as the most important parameter and if the effective opening (hydraulic openings) dosn’t calculate and it doesn’t taked into account, the cubic law will be deviated and rough surfaces caused more reduces in the flow rate fluid than the smooth surfaces of the rock mass.