A　unified　viscoplastic　constitutive　model　coupled　with　a　physically-based　damage　variable,　is　proposed　to　capture　the　cyclic　mechanical　behavior　and　microstructural　evolution　of　the　material　at　elevated　temperature.　The　mechanical　strength　can　be　reduced　by　the　decrease　in　the　dislocation　density,　the　coarsening　of　the　martensitic　lath　and　the　loss　of　the　martensitic　structure　under　cyclic　loading.　The　proposed　physically-based　damage　variable　is　driven　by　the　evolutions　of　dislocation　density　and　martensitic　lath　width.　The　good　com-parisons　with　test　results　mean　that　the　proposed　model　can　reasonably　model　the　cyclic　elastic-viscoplastic　constitutive　behavior　of　the　material　at　high　temperature.