A modification from classical gradient-dependent plasticity that is applicable to the material with uniform grains in size is presented by use of mixing law. For the material with different grains in size, shear band is analyzed. It is found that the size and content of grains influence the distribution of local plastic shear strain in shear band and the thickness of the band. In triaxial compression, an expression for the post-peak slope of differential stress-strain curve of rock specimen that is composed of the material with different grain diameters is proposed. The slope is influenced by the mechanical properties, geometrical properties and content of grains with different sizes as well as the shear band inclination. The present theoretical results are compared with the previously experimental results for granular material with different coarse grain percentages at different confining pressures. The validity of the proposed analytical model is verified.