Traditional horizontal-axis wind turbines are mainly designed as upwind configuration. In order to avoid blade strikes, the rotor blades have to be positioned far enough away from the turbine tower and have to be designed as inflexible as possible. In addition, a complicated yaw control system is required to keep the turbine rotor facing the incoming wind. Due to these drawbacks, the turbine in downwind configuration is proposed to overcome these disadvantages because, first of all, rotor blades can be designed more flexible since there is no danger of blade strikes, and secondly, yaw control system could be eliminated if nacelle is designed appropriately to follow the incoming wind direction passively. In the present study, a comparative experimental investigation was conducted to quantify the aeromechanic performance of a downwind turbine (DWT), in comparison to that of a traditional upwind turbine (UWT). The thrust coefficient of the DWT model was found to be increased slightly in the time-averaged quantity, but have a significant augment in the fluctuations. Due to the shadow effect, the power outputs of the DWT model was found to be decreased by 3.2% when they were operated in a same atmospheric boundary layer (ABL) wind. In addition, a high-resolution particle image velocimetry (PIV) system was employed to characterize the ensemble-averaged and phase-locked wake flow structures to quantify the turbulent flow characteristics in the turbine wakes. The velocity deficit in the lower half turbine wake for the UWT case was found to be greater than that of the DWT case at the location of X/D < 1.0. The higher wind load fluctuations for the DWT system were found to be correlated well with the higher TKE distributions in the turbine wakes. The phase-locked PIV measurements illustrated that the wake regions can be divided into four zones, which are dominated by the vortices shedding from different turbine components. The detailed flow field measurements were correlated with the dynamic force and power measurement data to elucidate the underlying physics.