The design method of a vortex pump has not been established as yet, because the flow behaviors in the impeller and chamber are very complex. Therefore further investigations are necessary for a vortex pump. For the purpose of a theoretical analysis, at first, an axisymmetric flow model has been deduced from the detailed measurements of the flow conditions. Next, on the basis of this flow model, the flow patterns in impeller have been resolved using a blade theory of an impeller at each flow surface of revolution. Predicted values by the theory have been compared with the experimental results, and good agreements have been obtained. On the other hand, a theoretical analysis using the equations of continuity, motion and angular momentum balance has been developed to predict the flow patterns in vortex chamber. The above procedures enables a theoretical prediction of the performance of a vortex pump.