Expander　is　a　key　components　in　Organic　Rankine　Cycle　(ORC)　system.　In　order　to　improve　the　performances　of　single　screw　expander　in　large　expansion　ratio　conditions　and　give　consideration　to　normal　conditions,　a　new　idea　called　＇increasing　the　built-in　volume　ratio　appropriately　and　converting　single　screw　expander　into　double-stage　machine　in　large　expansion　conditions　by　utilizing　the　discharge　velocity　of　screw　grooves　was　proposed.　By　implementing　this　idea,　a　new　prototype　of　single　screw　expander　was　designed　and　manufactured.　Then　the　prototype　was　integrated　into　an　experimental　ORC　system　with　R123.　Experiments　were　carried　out　to　analyze　the　characteristics　of　the　developed　single　screw　expander　and　the　ORC　system.　The　results　show　that　the　maximum　expander　shaft　power,　shaft　efficiency,　isentropic　efficiency,　volume　efficiency　and　expansion　ratio　were　8.35　kW,　56%,　73%,　83%　and　8.5,　respectively.　In　addition,　under　expansion　losses　seem　to　be　eliminated　by　the　new　structure　of　the　expander.　It　was　also　found　that　12-17%　of　the　expander　power　was　consumed　by　the　circulation　pump,　of　which　the　measured　efficiency　was　between　20%　and　31%.　A　maximum　ORC　efficiency　of　7.98%　was　achieved.　(C)　2016　Elsevier　Ltd.　All　rights　reserved.