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Pump performance depends on the outlet flow of the impeller. A method of surface singularities for core flow in the centrifugal impeller, combined with an integral method for a boundary layer, would explain the mechanism of the performance change with cutting the outlet edge of the impeller blades down. This method is applied to flows in the impellers with various cut downs of the blade edge, and then the calculated results are compared with the experimental ones. Both results are shown to be in quantitatively good agreement. On the influence of cutting the blade edge on the outlet flow, it is indicated that the cut of the pressure surface results in the decrease of relative flow angles with the decrease of radial velocity in the core flow, while that of the suction surface results in only decrease of the radial velocity. The change of the flow separation region due to the cut on the suction surface, however, combines to cause the pump performance to deteriorate.