Abstract: In order to explain the mode Ⅰ and Ⅱ of rock fracture under compressive loading, a stress model of microcracking element is presented on the basis of damage properties in the tip of main crack. Through studing the distribution law of stress intensity factors of branching cracks in various orientation, a new criterion of fracture mode and failure of mixed crack of rock subjected to compressive loading, expressed by ratios of stress intensity factors and fracture toughnesses, is obtained, which can predict not only mode Ⅰ, but also mode Ⅱ of fracture. Based on it, the geometrical and loading conditions of mode Ⅱ of fracture are given and discussed in detail.The results are in good consistent with experiments.