Abstract: Based upon shear strain gradient plastic theo ry of rock deformation localization,it is assumed that the failure pattern of ro ck in the shear bands is shear failure.A model of plastic shear strain gradient is obtained.Under the con dition of uniaxial compression of rock specimen,the inclination of shear band,p arameter k₁ and k₂ have an influence on the relationship of stress a nd strain in t he strain softening stage.The influence has been modeled.The results show that t he proposed model can be used in a wide range and the theoretical re sults are in a good agreement with many experimental results.Localization is th e reason for size effect.