The　unsteady　inlet　guide　vane　(IGV)-impeller-diffuser　interaction　in　a　centrifugal　compressor　is　investigated　numerically　and　experimentally　under　different　IGV　prewhirl　angles.　The　influence　of　IGV　prewhirl　angle　on　the　flowfield　and　the　unsteadiness　of　the　impeller　and　diffuser　are　studied,　and　the　mechanism　of　the　unsteady　IGV-impeller-diffuser　interaction　is　explored.　Results　show　that　quite　fair　quantitative　agreements　are　achieved　between　the　predicted　and　measured　flowfield.　The　IGV　wake　can　decrease　the　unsteadiness　on　the　impeller　and　diffuser,　and　the　reduction　of　the　unsteadiness　caused　by　the　IGV　is　a　function　of　the　IGV　prewhirl　angle.　The　unsteadiness　in　the　diffuser　with　negative　IGV　angles　is　larger　than　that　for　positive　prewhirl　angles.　The　low-frequency　oscillations　inside　the　flowfield　are　induced　by　the　IGV.　When　IGV　prewhirl　occurs,　the　basic　frequency　of　the　unsteady　flow　in　the　vaneless　diffuser　is　mainly　half　of　the　blade　passing　frequency　of　the　impeller.　In　practice,　the　IGV　prewhirl　angle　can　be　adjusted　to　avoid　unsteady　vibrations　in　some　off-design　conditions,　with　reliability　and　performance　improved　simultaneously.