In order to improve the performance of a Wells turbine, the effects of the gap between rotor and guide vane and solidity of the guide vane have been investigated by model testing. The results have been compared with those of the case without guide vanes. It is found that the overall characteristics of the turbine are considerably improved by the guide vanes. Furthermore, a suitable choice of design parameters such as the gap and the solidity of the guide vane has been suggested.