Abstract: A shear strength criterion of jointed rock masses, taking the contribution of joints, the contribution of rock bridges and the contribution of undutant angle made by adjacent joints into account, is developed. Further, some phsical constrants at extreme stress levels are discussed during the shear failure process of joints and jointed rock masses. Moreover, the parameters of the criterion may he random variables, so it can be used to analyze the failure probability of random models and the risk of rock engineerings.