Wind energy is the fastest growing energy in the world today. Vertical axis wind turbine (VAWT) is an effective way to benefit from this energy. So that, the present work aims to improve the performance of Darrieus straight bladed rotor. For improving the performance Darrieus straight bladed rotor, six aspect ratios and four chord lengths of Darrieus rotor are experimentally selected in an attempt to improve the performance of Darrieus wind turbine. The experimental results in the present work are obtained using three blades of a new airfoil shape design which is designated asEN0005 at different wind speeds. The comparison between six aspect ratios at the same wind speed and chord length showed that, the rotor with aspect ratio 0.833 and chord length 18 cm produces the maximum power coefficient.