Process zone is recognized as one of the dominant variable in the fracture process of rock, which is a region of damage around the crack tip mainly effected by character of rock such as size and brittleness. This zone has a key influence on the strength of rock and deformation pattern of a structure. Therefore, investigate size of the process zone based on different rock properties is a very feasible method in study of rock fracture. In this paper, a new bonded particle model (BPM) was used for simulation of the rock material. Macroscopic and microcosmic parameters were calibrated by uniaxial compressive, tensile and three point bending test. To examine the item of structure composed of rock with different size and brittleness, few groups of four point bending simulation tests are presented, with a detailed measurement of process zone. As the significant factors influencing the size effect, the length and width of the process zone were determined by a numeric statistics method. Base on analysis of size of process zone, it is shown that when the brittleness is low, seems there is no size effect exits, but as the material turns brittleness to ductile, the length and width of the process zone is become more dependent on the specimen size. The analysis of the numerical results indicated that the discrete element method with a bonded particle model is able to mimic the problem of rock fracture.