The reliability analysis in this paper is implemented directly starting from the basic variables of the HoekBrown criterion. A limit state equation, in which the basic variables of the HoekBrown criterion are contained, is built by introducing the factor of safety of slope. The calculation procedures of three methods used in the reliability analysis of this paper are analyzed in detail. The stability analysis for a cut slope shows that the reliability results obtained by three methods have little differences. This paper also suggests a simple and fast method for estimating the reliability of slope stability.