Appropriate frequencies for burst wave of a dielectric barrier discharge plasma actuator at a reference voltage are investigated by the low speed wind tunnel experiment for the airfoil separation control. Here, a reference voltage is defined as the minimum voltage for controlling a reference condition, and the reference voltage is adopted for each experimental conditions to eliminate errors caused by the degradation effect of the plasma actuator and individual variability. All the experiments are conducted with the flow condition Rec = 6.3 × 104 and α=12. Time-averaged pressure around the surface of the actuator- applied NACA0015 is measured. The results show that F+ of higher than 1 is more effective for separation control and appropriate F+ region considerably changes when BR and n are changed.