The　performance　of　expander　was　the　key　problem　to　realize　the　practicality　of　low　temperature　heat　source　Organic　Rankine　Cycle　power　generation　system.　Volumetric　efficiency　was　the　important　index　to　evaluate　the　performance　of　volumetric　expanders.　In　this　paper,　the　preliminary　work　of　the　single　screw　expander　performance　influenced　by　volumetric　efficiency　was　carried　out,　and　the　single　screw　expander　prototypes　made　by　ourselves　were　tested.　The　result　indicated　that　volumetric　efficiency　can　improve　the　performance　of　the　expander,　but　cannot　direct　reflect　the　working　condition　of　leaking　gas.