Existence of rock microcracks has great influence on rock shear fracture under compressive loadings. However

the effects of microcrack geometries and confining pressures on shear fracture angle are rarely studied in rocks. In this study

an improved expression of stress intensity factor containing crack angle effect is derived

which is based on the proposed expression at crack tips in Ashby’s microcrack model. Using this improved stress intensity factor

the constitutive relationship between crack growth

strain and stress is obtained

and it predicts the peak strength in rocks. Coupling this constitutive relationship at state of peak strength and Mohr-Coulomb failure criterion

the correlations of internal friction angle

cohesion

shear strength and damage are obtained. Effects of confining pressure

initial crack size

crack angle and friction coefficient on shear fracture angle are discussed. The rationality of theoretical model is verified with experimental data. The results show that internal friction angle

cohesion and shear strength decrease with increment of damage; shear fracture angle increases with the increment of confining pressure and friction coefficient

or the decrement of initial crack size; shear fracture angle decreases firstly

and then increases with increment of crack angle.