Abstract: WT5”B1Based on the basic mechanical properties of geomaterials,it is proved that Drucker postulate and the classical mechanics of plasticity are not suitable for geomaterials.The basic problems of constitutive theory of geomaterials are discussed.Based on strict theoretic analysis,the following have been verified:the single yield surface model which is based on classic theory of plasticity would be unsuitable to geomaterials whether the associated flow rule is applied or not;the yield surface of geomaterials would not be unique,and its number will be same as that of the freedoms of plastic strain increment;and the yield surface might not be convex;and the associated flow rule is unsuitable to geomaterials.LIUYuan xue(DepartmentofCivilEngineerin