Hydraulic Fracturing has been used successfully in the oil and gas industry to enhance oil and gas production in the past few decades. Recent years have seen the great development of tight gas, coal bed methane and shale gas. Natural fractures are believed to play an important role in the hydraulic fracturing of such formations. Whether natural fractures can benefit the fracture propagation and enhance final production needs to be studied. Various methods have been used to study the effect of natural fractures on hydraulic fracturing. Discontinuous Deformation Analysis (DDA) has been used to couple with a fluid pipe network model to simulate the pressure response during hydraulic fracturing. In this paper, the effect of natural fractures on hydraulic fracturing will be studied. In particular, the effect of several physical properties on fracture initiation and propagation will be addressed. They include rock joint properties, joint orientations and extensions. It will be shown that DDA is a promising tool to study such complex behavior of rocks.