Abstract: Grain breakage is a common physical phenomenon in nature. Rockfill materials may break up seriously upon high stress level of loading. Based on the thermomechanics and micropolar theory, a micromechanical model for granular matter is formulated considering the grain breakage. Three grains with the same radius bonded together are taken as an assembly of crushing grain to simulate the grain breaking process with the influence of lateral stress. By introducing the grain breakage criterion and using the homogenization theory, the stress-strain relationship in particle scale is established, and the stress and strain are modeled with breakage occurring in the process of loading. The computed results demonstrate that the proposed micromechanical model can simulate the increase of stress before grain breakage and stress dropping at occurrence of grain breakage.