Instability in slopes is one of the most common ground hazards in civil and mining projects. The main factor in occurrence of many instabilities in slopes is the dynamic forces due to blasting and earthquakes, that every year creates heavy financial and human being damages in various parts of the world. Many cases of falling the rocks and landslides due to seismic ground motions exist that this issue highlighted the importance of dynamic evaluation of slopes stability in earthquake prone areas. Therefore one of the most sensitive and most important problems in civil and open pit mining projects, is the dynamic stability analysis of rock slopes overlooking them. Because of earthquake is considered as an important factor in slopes instability and also Iran is a country that located on a seismic zones and according to the importance of dams and lateral structures as sources of water and electricity supply in every country, in this study, it is investigated the dynamic stability of rock slopes overlooking the intake structure of Bakhtiary Dam Powerhouse using the numerical simulation and with 3DEC software. As in numerical modelling using the Distinct Element Method (DEM), state of creation the discontinuities in model will have much effect on conclusions of modelling and blocks size that is formed due to the intersection of discontinuities in rock mass which have much effect in behavior of rock mass. In this research using a quantitative viewpoint of Geological Strength Index (GSI) that in it GSI determined based on tow factor, block volume and joint condition, and has been suggested most appropriate state of joint sets creation in numerical model that is block size generated have a high matching with block size in reality. After selecting the appropriate method in creation of discontinuities, it is implemented the static analysis of rock slopes overlooking the intake structure. For this purpose, initially the appropriate slope angle is selected in the middle wall of intake structure. Then, the values and history of displacements in all three wall of intake structure are checked and it has been found that right wall and middle wall of this structure from the aspect of static is stable, but is unstable the left wall of this structure. Finally by selecting the appropriate support system, this wall is also stable. Then in order to dynamic analysis of rock slopes the studied, According to the conditions of seismotectonics site and purpose of the analysis that is evaluate the stability of the model against MDE earthquake, it is selected the horizontal acceleration history of karebas earthquake after the scaling of this accelerogram for site and applying to base of the model, the dynamic analysis under this earthquake is implemented. It has been found that all three walls of this structure are unstable in level of MDE earthquake. Finally, it is suggested the appropriate support system for stability of rock slopes overlooking the structure under MDE earthquake.