Birds are known for their extraordinary agility, maneuverability, flexibility and endurance during their flight, even under some adverse flying conditions. Bird wings have been the most inspirational element, attracting the attention of researchers to reveal the underlying physical mechanism of lift production as well as to apply the results into the artificial flying vehicles. This paper presents a systematic experimental investigation on a passive flow control of a NACA0012 airfoil using real feather flap which is installed on the suction or pressure surface. The focus of the present study is to determine the major role of a real feather flap in the aerodynamic performance of a NACA0012 airfoil at small attack angles (α). The feather flap width w and its installation position x are varied from 0.27c to 0.8c and from 0.0 to 0.2c, respectively, where x is measured from the leading edge of the airfoil, and c is the chord length of the airfoil. Detailed particle image velocimetry (PIV) measurements are conducted to understand the origin of the aerodynamic benefits introduced by the feather flap. The flap mounted on the suction side may have a positive impact only at large α, beyond the stall. On the other hand, when mounted on the pressure side, the feather flap is proved to be beneficial to improve the aerodynamic performance of the airfoil at small α (=  -4° to 8°). The lift C and lift-to-drag ratio C /C are enhanced by 186% and 72%, respectively, for w  =  0.53c, x   =  0.2c at α  =  2°. Time-averaged and instantaneous vorticities, time-averaged streamwise velocity, and lateral velocity around the flapped airfoil weaken, decrease and increase, respectively, compared with those around the plain airfoil, which are attributed to the increased C and C /C .