The anisotropy and size effect of a rock joint have a great influence on its mechanical properties. Considering the relationship between these two features is of great significance to the evaluation of rock mass stability. In this study

in order to investigate the anisotropic distribution of rock joints at different scales

the anisotropic variation coefficient AVC3D is proposed which considers the three-dimensional morphology parameter of orthogonal directions. The AVC3D of four natural joints at ten different sampling sizes are measured based on the progressive coverage statistical method

and the size effect law of the AVC3D is obtained. The conclusions are as follows: the AVC3D reduces to stable with the increase of the rock joint size

and there is a good negative logarithm function relationship between them. The normalized statistical average values of the AVC3D and the normalized size of the rock joints show a good linear function

indicating that the AVC3D has a fractal structure. The fractal dimension D can be achieved using the regular statistics of the size effect of the AVC3D. This method reveals the mechanism that the maximum effective dip angle and the roughness coefficient C

which affect the directional change of the structural surface features

tend to be more stable with the increase of sampling size. It shows that once the rock joint reaches the threshold of the anisotropic size effect

the stable anisotropy will appear.