Abstract: To investigate the effects of natural fractures on the hydraulic fracture propagation of underground coal seams， a twodimensional model of hydraulic fracture intersecting natural fractures was built， based on which， the laws of fracture propagation and the failure mechanisms of natural fractures were explored by applying theoretical analysis and numerical simulation. The research suggested that the angle of interaction， the horizontal differential principal stress and the development degree of natural fractures are the three main factors that affect the directions of fracture propagation. In the case of low horizontal differential principal stress and low angle of interaction， the propagation of hydraulic fractures is likely along the natural fractures due to shear failure; on the contrary， in the case of high horizontal differential principal stress and high angle of interaction， the propagating fracture is likely to cross the natural fractures. Smallsized natural fractures have little or no effect on the directions of hydraulic fracture propagation whereas the propagating fractures move along the natural fractures when they are bigger in size.

Key words: hydraulic fracture, coal seam, natural fracture, horizontal differential principal stress, angle of interaction, fracture size