In this study, aerodynamic characteristics of a horizontal axis wind turbine (HAWT) were evaluated and discussed in terms of measured data in existing onshore wind farm Five wind turbines (T1, T2, T3, T4 and T5) were selected, and hub -height wind speed, UD, wind turbine power output, P and turbine rotational speed, Q data measured from these turbines were used for evaluation. In order to obtain characteristics of axial flow induction factor, a, power coefficient, C-p, thrust force coefficient, CT, thrust force, T and tangential flow induction factor, a', Blade Element Momentum (BEM) theory was used. According to the results obtained, during a year, probability density of turbines at a rotational speed of 16.1 rpm was determined as approximately 45%. Optimum tip speed ratio was calculated to be 7.12 for most efficient wind turbine. Maximum G was found to be 30% corresponding to this tip speed ratio.