Abstract: The size effect is one of the most important factors of structural plane, which has a significant impact on the physical and mechanical properties of rock mass. The experiment was conducted to study the effect of size effect on the strength and deformation of the structural plane of rock mass by scanning the core of rock specimens, modeling with data and using numerical simulation of direct shear tests on different sizes of rock samples. Meanwhile, an in-depth study of the strain curve of the structural plane under the effect of shear stress and substantial properties of shear deformation of different structural plane sizes was conducted by taking the direct shear tests under normal stress, the shear strength of structural plane had substantial correlation with its size effect, then the shear strength of empirical formula was established. Results indicated that the shear strength of structural plane increased with its increase in size, and began to stabilize when reaching a certain size, then the critical size was defined for representative elementary area. When determining the shear strength of structural plane, the structural plane size shall not be less than the area of representative elementary area. In this article, area of representative elementary area was 0.384 8 m².