Abstract: Based on the selfsimilar model of microcrack growth,the localization of strain and damage,and complete stressstrain relation for mesoscopic heterogenous rock under triaxial compression with moderate or low lateral compressive stress are investigated.It is showed that the complete stressstrain relation includes the stages of linear elasticity,nonlinear hardening,and strain softening. The behaviour of strain softening is due to localization of deformation and damage.The constitutive model,which analyse localization of strain and damage,is distinct from the conventional model. Theoretical predictions have shown to be consistent with the experimental results .