In the framework of critical state soil mechanics, a simplified hardening law is proposed to describe the change between subloading surface and the normal consolidation surface with plastic deformation based on subloading surface concept proposed by Hashiguchi. The characteristics of overconsolidated clays, such as stressstrain relationship, strain hardening and softening and stress dilatancy are simulated, and the results are in accordance with the data from triaxial drained compression test. Numerical model is also used to predict the stressstrain relationship in the isotropic consolidation condition and the stress paths in the undrained triaxial compression tests.