Abstract： Plastic strain gradient increment constitutive model in small strain plastic deformation is established by introducing the internal length of material to yielding function. The analytic expression of equivalent strain of rock under uniaxial compression and two-dimensional plane stress is presented. The relational formula between strain localization band width of rock and its inclination angle in plane stress is derived based on Mises’ yielding criterion. The distribution range of strain localization band is discussed and the forming conditions of maximal and minimal values are presented. I and II type deformation conditions of rock are discussed and the critical angle of failure is further derived. Detailed analysis is made on I type deformation of rock. The changing range of the inclination angle of strain localization band under different v and E／l is discussed. At last，by using the conclusion from the strain gradient plasticity theory and the Coulomb-Navier criterion on material failure，the contrast studies are made and reasonable explanations are given on coefficient of internal friction.
