We applied a wind acceleration device (wind-lens) to Vertical Axis Wind Turbines (VAWT), and examined power augmentation effects by wind tunnel experiments. We have investigated the optimum parameters of the wind-lens configuration. A wind-lens consists of a diffuser and brim. The increase in diffuser semi-open angle makes it possible to generate higher power output over a wide range of tip speed ratios. There is an optimum semi-open angle of diffuser. The inlet enhances power augmentation over a wide range of tip speed ratios. The best location of a wind-lens in the streamwise direction is in the center of VAWT. The curved surface type diffuser is more effective for power augmentation than flat panel type diffusers. The wind-lens demonstrates power augmentation with a curved surface diffuser by a factor of about 2.1 compared with a bare wind turbine.