Abstarct: The ability to inject rock mass, in particular, the injection of rock ponds in dams, depends on a variety of factors that can be divided into three categories, geological factors, technical factors and characteristics of injectable grout. One of the most important issues in constructing a dam, is the sealing of the dam by the injection operation to prevent water from penetrating in the dam. The control of water leakage from the dam is important both economically and in terms of the stability of the dam. Therefore, in this study, the intrusiveness of the rock mass is investigated, focusing on the case study of Rudbar Lorestan reservoir pump system. To do this research, the scientific background was first reviewed and the basic and essential information was collected. In the next step, the characteristics of rock mass including opening and roughness, spacing, direction, length of effect (stability), grout intensity and characteristics such as the value of the modulus of the block, viscosity and grout pressure, and their effect on the flow rate and grout penetration rate has been studied. Then, by focusing on the case study of Rudbar Lorestan Dam, a separate fracture network model (DFN) of rock mass geometric properties was developed using statistical information analysis. Subsequently, using 3DEC's separate software, a 3D numerical model of the grout penetration into the rock mass is prepared. This software has the ability to analyze fluid flow among discontinuities. The validity of the numerical model was measured baxsed on the results of the Luzhan test and finally the sensitivity analysis for grout viscosity parameters and joint openings were investigated. The results show that grout penetration rate for the study of Roudbar dam is 1.5 meters. Grout viscosity has little effect on grout penetration, but joint opening has a direct effect on grout penetration. Also, anisotropy of rock mass has an important influence over grout penetration and orientation.