Abstract: Based on the framework of thermodynamics and critical state soil mechanics, a new α-β yield surface is formulated in the triaxial space. Based on the non-associated flow rule and bounding surface framework, the proposed model can simulate the correct elastoplastic responses within its initial yield surface for overconsolidated clay. By defining the dissipative function and back stress being functions of stress, the non-associated flow is naturally followed in the true stress space. The mapping center in the p-q space is selected as the projection on the hydrostatic axis of the intersection of critical state line and bounding surface, such that the contractive response on the ‘wet side’ and dilative response on the ‘dry side’ can be guaranteed if an appropriated dilatancy function is defined. Comparisons are made between the experimental data of three typical clays under undrained condition, and a full agreement indicates the simulative capability of the proposed model.