Abstract:When rock-like materials are tested under triaxial compression,two kinds of failure may happen,that is,ductile and brittle failures. The essential reason for rock failure is the relative displacement among mineral particles making up rock. Generally,rock failure can be either tensile or shear,which depends on the predominant mechanism when failure happen. The maximal tensile strain criterion can predict rock failure in tension,but is difficult to explain the effect of crack angle. Rocks are generally anisotropic. The principal strain axes are usually different from the loading axes. Therefore,the strains in the directions of loading axes may not be suitable to describe rock strength. The strain-based strength criterion considering tensile strains in three axes can describe rock strength. And it can describe the crack angle and its transformation. The theoretical analyses and laboratory tests presented in this paper validate this conclusion. With the increasing of the intermediate principal stress,the transformation of the strain failure criterion is of the same pattern as the transformation of the compressive strength and the maximum tensile strain criterion.