Abstract: The concept of equivalent height difference is proposed based on the two failure modes of the rectangular- shaped asperities to reflect that the different geometric parameters of the asperities have different contribution to the shear strength. The distribution of the equivalent height difference in different shear directions can reflect the direction- al character of shear strength. Then,a 3D roughness parameter system is developed,which consists of two roughness parameters. One is the average equivalent height difference,which represents the contribution of asperities with differ- ent failure modes to the strength. It also can describe average undulation directivity. The other one is fractal dimen- sion,which characterizes the relationship of roughness with different scales and can describe the information of the joint surface comprehensively from the scale. Finally,the new method is used to characterize the roughness of natural rock joint. Research results show that the new method can well reflect the 3D morphology of joint and it can also de- scribe the anisotropy of roughness and overcome the influence of the sampling interval. The new parameter system has a clear physical meaning,which offers the foundation for further studying the shear strength.