Abstract: The LC (loading-collapse) yield surface and the SI (suction increase) yield surface in the BBM model proposed by Alonso for unsaturated cohesive soils are first unified as a smooth yield surface.An elastoplastic constitutive mode with the unified yield surface is then presented in this investigation.One of advantages using a unified yield surface is that the coupled effects of LC yielding and SI yielding can be simply analyzed,i.e.the stress paths can be simulated when the net stress and the suction change at the same time.Otherwise,it brings the convenience for numeric simulation because of no corner on the yield surface.The proposed model contains 14 model parameters.Among which,12 parameters are associated with the physical and mechanical properties as well as yield characteristics of unsaturated soils,and 2 parameters are related to the plastic hardening of materials.All of these model parameters can be determined using the triaxial isotropic compression and shear tests with controlled suction.The numerical simulations of the model are compared with experimental data obtained from this investigation and from some published research results.It is shown that the model simulations accord with these test results well,and that the proposed model is thus quite reasonable and of certain practical value.