The time mean characteristics and the torque variation of the straight-bladed vertical axis wind turbine attached with various end plates were examined in wind tunnel experiments, and they were associated with the effect of the wing tip shape on the performance of the wind turbine. In the low tip speed ratio range, the time mean torque and the time mean power coefficients are reduced by attaching end plates. In the high tip speed ratio range, the time mean torque and the time mean power coefficients are increased by attaching end plates except for certain condition. Their maximum increase takes place at the condition that end plates are attached horizontally toward the side of the rotating axis. The effect of end plates was discussed by comparing the variation in the phase averaged torque coefficient. For each condition at the low tip speed ratio and the high tip speed ratio, the phase averaging analysis is able to specify the particular phase concerned to the significant change of the time mean torque coefficient and give the physical interpretation of the phenomenon.