A 200RT centrifugal refrigerant compressor had been totally designed and analyzed.The objectives of the design are high efficiencies at full load and partial load conditions.The design procedure includes a conceptual design,a preliminary design,detailed design and flow analysis.The thermal cycle and rotational speed are determined in conceptual design.The performance curve is predicted in preliminary design; complete impeller blade profiles and the geometry of vaneless diffuser are obtained in detailed design; the flow fields of impeller and vaneless diffuser are simulated by CFD.The design rotational speed is 24000rpm.The number of impeller blade is 8.In this design case,it is not good for pressure recovery when the width of vaneless diffuser is too wide or too narrow.Small vaneless width causes large friction loss at full load condition while large vaneless width causes recirculating flow at partial load conditions.The results of this study show that an adjustable vaneless width can obtain a wide high efficiency region covered full load and partial load.