Abstract: Using the technique of collocated grid scheme and sliding mesh,coupled with standard k ε turbulence model,the computation of the unsteady flow in a centrifugal compressor stage model due to impeller/diffuser interaction is performed by a pressure based algorithm under the general curvilinear coordinate system.The computation results show that,the impeller/diffuser interaction has unsteady effect on the whole flow field of the stage.The unsteady component of the velocity is marked in the gap between the impeller and diffuser,while vanishes quickly in the blade channels of the impeller and diffuser.However,the pressure unsteadiness due to the impeller/diffuser interaction remains obviously throughout the blade channels.The results simulate the impingement of the flow from the impeller on the diffuser blades adjacent downstream,and,the transportation of the vortex series at the diffuser inlet and its interaction with the diffuser blades,which may result in noise.