The behaviors of geomaterials before destroy can be reflected by the representative volume element; but when the high strain gradient occurs (such as the strain localization occurs)

the representative volume element in which the strain and stress ate nonlinear

cannot reflect the mechanical character of geomaterials entirely. A plasticity gradient volume element (PGVE) which can describe the local deformation is proposed. The start-up mechanism of strain localization is analyzed; when the hardening modulus (or softening modulus) changes from positive to negative

the deformation pattern will change from uniform pattern to localization pattern.