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.