The flow field in radial impeller of a centrifugal pump has been predicted by assuming two dimensional ideal flow. The Poisson's equation, describing the flow field, is solved numerically by relaxation method, for both circulatory and flow fields. The theoretical impeller head was calculated as the mean value of the head generated by each stream tube in the impeller flow pattern. In spite of neglecting the boundary layer effect, the experimental results show a satisfactory agreement with the theoretical results, at part loads. this shows that the main predominant factor controlling the pump characteristics at part loads is the vortex formation in the impeller channel, and that the boundary layer has a minor effect in that operating region.