In order to evaluate the stress-strain behaviours of undisturbed ”Masado” which is one of unusual soils in Japan because of its collapsibility, the mechanical properties are investigated on the basis of the triaxial test results and the isotropic elastoplastic constitutive model for ”Masado” is presented. The proposed model contains eight parameters which are easily determined from a few conventional triaxial tests. It is proved that the proposed model can reasonably evaluate the depedency of the stress-dilatancy and strength properties of undisturbed ”Masado” on the confining pressure and the degrees of weathering.