Abstract: To estimate the influence factors on the natural rock discontinuity roughness, taking an in-situ large scale rock collected in Wulong County, Chongqing City as an example, the variation rules of roughness under different sampling directions, sizes and precisions are analyzed by using the 3D laser scanning technology, and Grasselli’s method is improved through discretization of rock discontinuity into a large amount of quadrangular tiny planes rather than triangular planes. The results show that the natural rock discontinuities considered herein are characterized by obvious anisotropies, positive scale effects and interval effects. Therefore, the sampling direction with respect to roughness data acquisition should be kept the same with the shearing direction (direction of rock mass movement) in the procedure of roughness evaluation. In addition, the sampling size is specified to be equal to or larger than the effective sampling size, while the sampling interval should be equal to or less than the effective sampling interval. For the convenient comparison among the assessed results by different methods, this kind of study should be performed under the same sampling parameters including sampling directions, sizes and intervals.