Abstract: The finite deformation theory and the Nakai’s sub-loading constitutive model are used to investigate shear band localization in plane strain model tests. A stable and fast converging algorithm for the numerical integration of isotropic elasto-plastic models is used. Under the precise control of local and global tolerances, an obvious X-shaped shear band pattern is obtained. During the formation and development of the shear bands, the iteration stability difficulties are observed twice. One happens prior to the critical state, and the other one is present around the critical state. After these numerical stability difficulties, the shear bands are accelerated to propagate and stretch out in four directions. It is also indicated that the iteration stability difficulties are deeply associated with the initiation and propagation of the shear bands.