The flow distortion at the impeller inlet of the multistage centrifugal compressor which employs three-dimensional impellers, and its effect on the overall compressor performance are studied experimentally by using a two-stage test apparatus. The flow distortion caused by the inlet casing which turns the flow from a radial to an axial direction are investigated. this experiment reveals that if the inlet casing of a low loss coefficient is used, the overall performance is almost the same as that obtained from an axial inlet stage. The flow distortion caused by the return channel are also investigated. The experiment shows that even if a flow distortion occurs at the second stage impeller inlet by the effect of the flow passage of the first stage, the overall performance of the whole stage can be predicted based on the single stage performance data obtained from the axial inlet test apparatus.