Abstract: Vague information widely exits in many aspects of the stability analysis of rock and soil slopes. By studying the vague factors in the slopes, a new approach is developed for the slope stability analysis using fuzzy sets,and a new fuzzy mathematical model is proposed to analyze the stability of slopes. In this model, the vague character of mechanical parameter,such as density, cohesion and frictional angle, is considered. For the convenience of computation, interval arithmetic is introduced to substitute fuzzy arithmetic. On the basis of the analysis of test data of rock and soil, a method to construct the membership functions is proposed according to the experience of specialist and a membership function for the evaluation of stability analysis of slopes is also constructed by use of statistical data. A new method used to evaluate membership degree of fuzzy safety coefficient and index for the evaluation of slope stability are also suggested in this paper. The case study indicates that fuzzy analysis method of slope stability is a more reasonable analysis method. 