The　selection　of　expanders　for　organic　Rankine　cycle　(ORC)　systems　is　very　important,　because　it　affects　the　performance　of　the　system　significantly.　Single-screw　expander　is　a　new　type　of　volume-type　expanders,　which　is　very　suitable　for　medium/small　scale　ORC　systems.　In　order　to　investigate　the　performance　of　single-screw　expander　in　an　ORC　system,　an　experimental　system　is　set　up　which　is　used　high　temperature　gas　as　heat　source　and　R123　as　working　fluid.　By　adjusting　the　torque　and　back　pressure　of　single-screw　expander　and　the　vapor　dryness　of　working　fluid,　the　performance　of　single-screw　expander　has　been　obtained.　The　power　output　of　single-screw　expander　is　5.46　kW,　and　the　total　efficiency　and　volume　efficiency　are　48.7%　and　80%,　respectively.　The　results　shown　that　under　the　same　heat　resource,　the　expander　ratio　of　single-screw　expander　increased　with　the　risings　of　torque　and　dryness,　but　it　decreased　with　the　increase　of　back　pressure;　the　volume　efficiency　decreased　with　increases　of　torque　and　dryness;　and　the　total　efficiency　of　single-screw　of　expander　decreased　with　increases　of　back　pressure　and　dryness,　and　with　increase　of　torque,　it　increased　at　first　and　then　decreased　after　the　maximum　is　gotten.　©,　2015,　Journal　of　Mechanical　Engineering.　All　right　reserved.