Particle breakage has a significant influence on the stress-strain and strength behavior of rockfill material. A breakage critical state theory (BCST) was proposed to describe the evolution of particle breakage. The breakage critical state line in the breakage critical state theory was correlated with the breakage factor, which was fundamentally different from that of the original critical state theory. A simple elastoplastic constitutive model was developed for rockfill in the frame of BCST. An associated flow rule was adopted in this model. Isotropic, contractive and distortional hardening rules were suggested in view of the particle breakage. It was observed that the proposed model could well represent the complex deformation behaviors of rockfill material, such as the strain hardening, post-peak strain softening, volumetric contraction, volumetric expansion, and particle breakage under different initial confining pressures.