The present research was carried out to summarize the effect of an attached flap at the trailing edge on a vertical axis wind turbine performance. Two models were tested in a wide range of initial flow velocity and blade pitch angles. The first model has two blades, and in the second model four blades were used. The pitch angle was varied from 15^o to 60^o. The torque and power coefficients were measured. A comparison with data obtained with the model without flaps is given. It is found that the torque and power coefficients increase with increasing the pitch angle up to  45^o and then decrease at the a higher value of the pitch angle. The attached flap reduces the flow separation, enhances lift, captures more wind energy, increases the rotor speed and hence increases the torque and power coefficients. The flap at a position L/C_f= 0.75 for the rotor has a pitch angle of 45^o gives an optimum value of the power and torque coefficients compared to the other values of L/C_f . At these conditions, an improvement of 7.14 % is achieved (for the rotor has two blades), and of 12.7 % (for the rotor has four blades) compared to the rotor without flaps.