Abstract: An essential feature of stress-strain behavior of soils different from that of metals is the cross effect of the mean normal stress on the deviatoric strain and that of the deviatoric stress on the volumetric strain. In addition, it has been shown by experiment data, that the direction of plastic strain increment vector is not uniq uely defined by the stress state, but also dependent on the direction of stress increment vector during shear. In order to better describe these mechanical properties of soils, a new concept of so-called partial yield loci is formulated. Oppositely, the yield locus in the usual mean is renamed as the general yield locus. According to this new concept, the yield of the volumetric yield locus introduces only the plastic volumetric strain; similarly, the yield of the deviatoric yield locus induces only the plastic distortional strain.The mechanical model based on this new theory satisfactorily explains the dependency of the direction of strian increment vector upon that of stress increment vector, and also explains the various behaviours of soils, such as dilatation-contraction and hardening-softening, as well. In addition.it has been shown, that the model is in accordance with I' iushin's postulate of plasticity and satisfies the requirement of uniqueness of solution in boundary value problems of elasto-plasticity.Finally,a special model for the normally consolidated clay is proposed.