The Savonius rotor was proposed in the early 1920's. Until now, there has been no systematic work on the optimum design configuration of Savonius rotors. Therefore, in the present work, various model tests were carried out in a wind tunnel, in order to ascertain the optimum design configuration of Savonius rotors. The effects of six design parameters on the aerodynamic performance of the rotors were experimentally determined. These were the rotor aspect ratio, the overlap and the separation gap between rotor buckets, the profile of the bucket cross-section, the number of the buckets, and the presence or absence of rotor endplates. In addition, the flow around the rotor was investigated by the flow-visualization method. From this study, the influence of important design parameters of the Savonius rotor have been investigated and the rotor configuration giving the maximum torque and power has been determined.