Boundary conditions have great impact on the stability of three dimensional geo-material, like slope, which is one of the oldest applications in geotechnical engineering. In order to analyze the impact of boundary condition to factor of safety of slope, the 3D stability analysis was then extended to the “rough-smooth” and “smooth-smooth” boundary. By studying slopes with the same geometric condition and shear strength parameters, the results show that the safety factors obtained from “rough-smooth” boundary are smaller than those got from “rough-smooth” boundary and bigger than those got from “smooth-smooth” boundary. The study also indicates that when the width to height ratio (W/H) of homogeneous symmetric slope satisfies certain condition, the “rough-rough” boundary equals to the “rough-smooth” boundary. The factor of safety and shape of slip surface got from these two kinds of boundary condition are of the same. Study results can give guidance for the real practice.