Abstarct: Slope stability analysis is one of the most important problems in the mining and geotechnical engineering. Ignoring the importance of this problem can lead to significant losses in terms of safety and economy. Selecting an appropriate method for analyzing the slope stability requires a proper understanding of how different factors influence the outputs of the analyses. Slope stability analyses are mostly performed by two-dimensional (2D) methods with some simplification assumptions. Whereas in the reality, all problems are three-dimensional (3D). In 2D analyses, inappropriate assumptions are used and ignored the effects of the more realistic 3D analysis. This cause differences between 2D and 3D analyses. In various studies, these differences are referred to as “3D-effect”, and are usually represented as the ratio of the 3D factor of safety (FOS3D) to the 2D factor of safety (FOS2D). This study, evaluates 3D-Effect caused by considering the real geometry, steepness of the slope and the changes in the mesh size as “dimensional effects”; and 3D-Effect caused by the changes in the rock mass strength properties (friction angle and cohesion) and the percentage of joints as “media effects”. To the aims of this study, numerical continuous modeling softwares, FLAC and FLAC3D, and numerical discontinuous modeling softwares, UDEC and 3DEC, were used to modeling and analyzing the slope stability of Tectonic Block 1 in old designed (the steep slope) and newly designed (the gentle slope) pits of Choghart Iron Mine. The results of this analyses were compared in terms of factor of safety and potential failure surface. In the first step, several 2D and 3D continuous analyses were performed to evaluating the mesh effect on the results of the analyses, evaluating the 3D-Effect caused by considering the real geometry and the mesh effect, and selecting an appropriate mesh size for the analyses. In the next step, number of analyses were conducted to assessing the effect of the strength parameters’ changes on the each 2D and 3D analyses. In the final step, some 2D and 3D discontinuous analyses were performed to evaluating the joint density effects on the results of the analyses and 3D-Effect. Results showed that the differences between the 2D and 3D analyses are considerable. Among the all considered parameters, the changes in the friction angle, particularly in the gentle slope, has a great influence on the changes in the 2D and 3D results, and also 3D-Effect.