Abstract: The artificial rocks specimen are made of gypsum powder in the laboratory and the surface structure is formed after the cutting.Then,the surface roughness of the structure is described by the digital gray images method.Secondly,the surface structure is divided into ten sampling windows according to five different programs with ranges from 100 Pixel×100 Pixel,200 Pixel×200 Pixel to 1 000 Pixel×1 000 Pixel.Finally,the values of SRv in all directions of the surface structure under different size are calculated by the SRv characteristics method,that is,the surface roughness is characterized by variogram.The results by each scheme show that 1) in the same direction,the smaller size is,the larger SRv value is.As the size increases,the SRv value decreases and then reaches a stable value.But in different directions,the threshold values of SRv are not the same.(2)In addition,the anisotropic characteristics are different among the different scale structural planes.Only by fully understanding the anisotropic characteristics of structure surface under different scales can accurately grasp the characteristics of size effect; Only accurate grasp of the size effect can effectively grasp the anisotropic characteristics.This understanding provides a new way to accurately grasp the mechanical properties of discontinuity.

Key words: Discontinuity, Size effect, Anisotropy, Roughness