Abstract: A simple and unified constitutive model for both clay and sand under three dimensional stress conditions is derived from the modified Cam clay model,taking the following two points into consideration.First,a transformed stress tensor based on the SMP (Spatially Mobilized Plane) criterion is applied to the Cam clay model.The proposed model realizes the consistency from the shear yield to the shear failure and the combination of the idea of the critical state theory with the SMP criterion for clay.Secondly,a new hardening parameter is derived in order to develop a simple and unified constitutive model for both clay and sand.It can not only describe the dilatancy from lightly to heavily dilatant sand,but also be reduced to the plastic volumetric strain for clay.The validity of the hardening parameter is confirmed by the test results of triaxial compression and extension tests on sand under the various stress paths.Only five conventional soil parameters are needed in the proposed model.