Abstarct: In the past, the rock slope stability analysis was usually carried out using graphical method or manual calculations, but today there are various analytical methods for rock slope stability analysis. These methods need to be determined according to the region conditions and the type of potential failure. Also due to the low level stresses in shallow parts of the ground such as slopes, rock mass behavior in these parts is controlled by discontinuities. So to study these instabilities, investigating the discontinuities shear behavior is necessary. Analyzing, designing and predicting the performance of structures which is made inside or top of rock masses, needs proper understanding of the geomechanical properties of the rock mass, including strength, deformability, cohesion and inside friction angle. In this study, the rock slope stability of Tehran-Shomal freeway first part was investigated using 3D diffencial element method. To analyze rock slope stability, at first geometric characteristics of the joint sets were determined using experiment results in references. Next, further experiments were carried out on mechanical properties of rock mass and joints. Then, to determine the overall rock mass behavior, some analyses were performed using Slide software. In the next step, the geometrical and mechanical model of the rock slop were built in the 3DEC software and the the rock slop stability was studied in different situations. Results showed that the rock slop is stable in usual condition considering different stability analysis methods. But in weathering conditions which cause rock mass shear strength reduction, rock slop is unstable. For stabilization of rock slope, the rock mass strengthening and anchor installing are proposed.