Abstract: Yield and failure behaviors of rock and soil masses were analyzed from the mechanical meanings.Some problems were pointed out when slope stability was analyzed with Mohr-Coulomb’s failure criterion.The intuitionistic explanations of the mechanism for formation of critical slip surfaces in slopes were given by adopting Mohr-Coulomb’s failure criterion and tension-shear composite failure criterion respectively.Several examples were given by using strength reduction finite element method.It was shown by field examinations and reconnaissance of many landslides that the practical slip surfaces could be explained by using tension-shear composite failure criterion.