Abstract: The mechanism of formation of shear band in cohesionless soils is presented and verified by a fully coupled FEM procedure. The numerical analyses show that the initiation of shear band is related to softening behavior of material, and upon the occurrence of shear band,there are two different shear modes exerted in the soil body: within the localized zone, the constitutive relationship is characterized by softening loading, while unloading takes place at the most parts far from it. Moreover, the volumetric strain within shear band is significantly larger than that beyond it, and critical state occurs locally at large global strain level.