Three-dimensional potential flow in a screw-type centrifugal pump was calculated in consideration of leakage flow through a tip clearance of the blade. The drop of the theoretical head due to the leakage is calculated, and this result agrees qualitatively with the experimental result. From the nemerical and experimental study on three impellers of different blade twist angles (360°, 540°, 720°), it is found that the drop of the pump head due to the leakage increases with increase in tip length of the blade. Next, effects of prerotation on pump characteristics were investigated by numerical simulation and experiments. In the case of the commercial impeller, it is found that the prerotation does not affect the amount of angular momentum at the impeller outlet. But the numerical results on the 360°twist impeller whose passage is wider than that of a commercial impeller show that the flow around the impeller outlet is influenced by the prerotation. It is found that the difference between the calculated theoretical head and measured pump head increases when the direction of prerotation is opposite to the rotation of the impeller.