Granular　materials,　such　as　rockfill　materials　and　carbonate　sand,　exhibit　complex　mechanical　behavior,　which　is　affected　by　particle　breakage　under　elevated　pressures.　In　this　study,　we　present　a　relative　breakage　model　based　on　mean　stress,　void　ratio,　and　strain.　We　further　consider　the　effect　of　relative　breakage　on　the　critical　state　line.　A　hypoplastic　constitutive　model　is　developed　by　incorporating　the　critical　state.　The　validity　of　the　constitutive　model　is　shown　by　comparing　the　numerical　and　experimental　results　of　drained　triaxial　tests.