Abstract: On the basis of systematic measurement, the fractal characters of rock joint surfaces and the mechanical behavior of the rock joints under shearing have been investigated. The study elaborates the effect of surface roughness characterized by fractal dimension D and the intercept A on the normal and shear deformation, and the shear strength of rock joints. The evolution law of roughness due to the surface damage in loading has been developed. The study shows that D and A are important and independent of each other in describing surface roughness of rock joints. The former reflects the irregularity of asperities on joint surfaces, while the latter is intimately related to the slope of waveness of rock joints. Fractal dimension D alone is insufficient for establishing the relationship between the surface roughness and the mechanical behavior of rock joints. Under most circumstances, the dependence of the mechanical properties of rock joints on the intercept A is greater than that on fractal dimension D.