Abstract: The orientation of fracture can be observed directly on observation window; however, if we determine the orientation and distribution features of the fractures from statistical data on observation window directly, there would be some errors. This is because the fracture which is vertical to observation window tends to intersect with observation window and appears more easily; on the contrary, the number of those that are parallel to the observation window will decrease as little appeared. According to the orientation of fractures and observation window besides the size of the observation window, we can infer the theoretical number of the fractures. Assuming the same of fractures occurrence, new fractures can be grouped according to the certain way, and the theoretical average occurrence of fractures can be determined.