It has important value of theory and practical significance to understand the mechanical characteristics of rock structural planes by taking direct shear tests for shear strength of simulation rock structural planes with different sizes (especially large sizes). Based on MSJ-DST

the direct shear tests of simulation rock structural planes in sizes 20 cm×20 cm

40 cm×40 cm

60 cm×60 cm

80 cm×80 cm and 100 cm×100 cm are taken under the normal stresses from 200 kPa to 1 000 kPa. Then

the characteristics of shear strength of rock structural planes with different sizes are studied. The results indicate that the stresses and deformation characteristics of simulation rock structural planes under different normal stresses are similar; and the peak shear displacements are generally fluctuated in the vicinity of a certain value. Under the same normal stress

the peak shear strength values of simulation rock structural planes with different sizes are fluctuated in the vicinity of a certain value; and the residual shear strength values rise slightly as the size increases. Under five grade normal stresses

the variations of peak shear strength and residual shear strength with different sizes are approximately same; and the ratio of residual shear strength value and peak shear strength value increases and tends to be stable as normal stress increases.