A　single　screw　compressor　is　widely　studied　as　a　promising　compressor　technology　applied　in　heat　pumps.　The　capacity　control　mechanism　is　required　for　the　compressor　to　satisfy　the　actual　heating　capacity　under　different　working　conditions.　In　this　paper,　the　frequency　conversion　regulating　mechanism　by　changing　the　motor　rotating　speed　is　employed　to　control　the　capacity　of　the　single　screw　compressor.　Experimental　investigations　on　a　single　screw　compressor　with　variable　speed　for　heat　pump　are　carried　out　under　different　working　conditions　to　research　its　performance.　The　tests　have　been　performed　using　R22　at　evaporating　temperatures　between　-9　degrees　C　and　3　degrees　C,　and　motor　rotating　speed　between　1710　rpm　and　2850　rpm.　The　heating　capacity　and　coefficient　of　performance(COPh)　COP　h　)　changed　between　47.19　kW　and　165.63　kW,　and　between　1.87　and　3.68,　respectively.　The　volumetric　efficiency　of　the　compressor　presents　a　linear　decrease　by　28.66%-36.04　%　for　different　evaporation　temperatures　when　the　motor　rotating　speed　decreases　from　50　Hz　to　30　Hz.　Results　show　that　high　motor　rotating　speed　has　a　positive　effect　on　the　volumetric　efficiency,　power　consumption,　heating　capacity,　and　COP　h　.