Improving　thermodynamic　efficiency　of　prime　movers　is　the　key　issue　for　efficient　utilization　of　low　temperature　heat　resources.　Because　of　many　good　characteristics,　the　single　screw　expander　is　perhaps　a　good　choice.　Precisions　in　manufacture　and　assembly　are　very　important　factors　to　the　performance　of　single　screw　expanders.　In　this　paper,　experimental　results　of　the　first　single　screw　expander　prototype　made　by　ourselves　was　analyzed　and　reported　for　the　first　time.　From　the　experimental　data,　the　power　output　of　the　prototype　was　about　5kW,　but　the　gas　consumption　rate　was　above　105kg/kWh,　so　the　shaft　efficiency　was　only　34%　or　so.　The　results　indicated　that　internal　leakage　was　very　serious.　Hence,　the　second　single　screw　expander　prototype　was　manufactured.　The　gap　between　gate　rotor　and　shell　of　the　new　prototype　was　reduced.　The　gap　is　about　0.02mm.　From　the　new　experimental　data,　the　mass　flow　rate　was　significantly　decreased.　However,　the　power　output　was　only　1.4kW.　So,　the　gas　consumption　rate　was　even　more　than　the　first　prototype.　The　reason　of　bad　experimental　results　may　arise　from　the　too　large　friction　between　screw,　gate　rotor　and　shell　in　the　case　of　small　gap.　So,　we　manufactured　the　third　single　screw　expander　prototype,　which　is　0.04mm　in　gap.　From　the　experimental　data,　the　maximum　power　output　was　about　4.5kW,　but　the　mass　flow　rate　was　decreased　sharply.　So　the　gas　consumption　rate　was　about　60kg/kWh,　and　the　shaft　efficiency　was　about　60%.　The　result　indicated　that　the　performance　of　the　single　screw　expander　was　remarkable　improved　by　optimizing　its　configuration.　Copyright　©　2012　by　ASME.