Fracture ratio is one of important parameters for evaluation of rock mass quality. The fracture ratio of rock is analyzed by digital image analysis with matlab. In this paper, the width of different place on fracture of rock is gotten, which is more important than average width of fracture for analysis characteristic of fracture. The area of fracture is directly gotten from the binary image of fracture and it can avoid the error of area of fracture due to average width of fracture. So the fracture ratio of area is more exacter. The practice shows that the method of digital image analysis is a good method for investigation of fracture ratio of rock.