Based on the statistical analysis of a large amount of measured data in slope enginering, a slope failure can be regarded as a fuzzy event. A fundamental fuzzy model for describing slope failure is established by using the theory of fuzzy probability measures in combination with the conventional safety factors of the slope. This model may be applied to predict the slope failure and to evaluate its stability. The calculation results through an illustration show that the critical value of the safety factor(F) for the rock slope should be greater than 1.15.