The purpose of the present study is to explain the shear behaviors of soils under threedimensional stress condition uniquely, by extending the concept of the Spatial Mobilized Plane (SMP) proposed previously. From the reconsideration of the former stress-strain model based on the SMP, new amounts of strain increments (dε_SMP^* and dγ<SMP>^*) which represent the normal and parallel components of the principal strain increment vector to the SMP are defined. Then, a new stress-strain model under shear is proposed on the basis of the idea that there exist unique relations among these new amounts of strain increments and the shear-normal stress ratio on the SMP. The validities of this stress-strain model and the failure criterion based on the SMP are confirmed using the results of the triaxial compression, triaxial extension and true triaxial tests on sand and clay. All the soil parameters of this stress-strain model can be easily determined using the conventional triaxial compression tests.