Abstract: The mechanical behaviors of soils are dependent upon the structure and the stress state. Based on the unified strength theory, the unified strength parameters are proposed by introducing Lode's angle into the unified strength theory. The critical state line is modified by considering the effect of the intermediate principle stress under the framework of the critical state concept. The effects of structure are introduced into the yield function of the modified Cam-clay model. The non-associated flow law is adopted. The equation for dilatancy stress is determined. A structural constitutive model that can reflect the effects of the intermediate principal stress is proposed. The model is verified by true triaxial tests of a clay. The results show that the proposed model can predict the stress-strain relationships well.