Estimation of stability of the soil body is the main objective in the design of structures and foundations. But the laws of mechanics, such as the equilibrium conditions and the boundary conditions are not observed in most methods used for estimating the stability of the soil body. To correct the noted disadvantages, the special method, in which all laws of mechanics are observed, is stated. Search for the most dangerous surface of failure is based on the variation principle. The proposed method is used to identify the impact of such factors like boundary conditions, the shape of the surface of failure, the law of normal stress distribution over the surface of failure on the estimating the slope stability. It is established that all these factors significantly effect on the estimating the stability of the soil body.