The stability of rock slopes is analyzed by considering the spatial variability of the rock materials based on theory of random fields. Both the cohesion and frictional coef cient along the main sliding surface are treated as Gaussian random elds, and the mean value, variance, covariance of the cohesion and frictional coef cient are determined. Spatial correlation lengths and the cross-correlation between cohesion and coef cient are obtained. Three-dimensional stability assessment of rock slopes is analyzed to obtain the factor of safety and probability of failure of rock slope. The numerical results show that the spatial variability of cohesion and coef cient of friction significantly affect the safety of rock slopes.