Abstract:The significant testing results on rock failure are obtained from the mechanical tests for a crystallinoclastic tuff under different strain rate of 10-5~1 s-1 and confining pressures of 0 and 88.2 MPa. Furthermore,the strength model considering the effection of strain rate and confining pressure,is proposed based on Mohr-Coulomb criterion. The model indicates that the failure of rock is a process in which there is a coupling between crack development and the slipping of cracked plane. The strain rate threshold is obtained from the test for strength catastrophy,and the mechanism of local second rupture of rocks is gotten clear.