Abstract: An array of 26 plasma synthetic jet actuators (PSJA) is flush-mounted on a NACA-0015 airfoil model to control the leading-edge flow separation at moderate Reynolds number (Rec= 1.7 × 10 5). The stall angle of this airfoil is postponed from 15. 5 ∘ to approximately 22 ∘, and the peak lift coefficient is increased by 21%. PSJAs exhibit distinctive separation control mechanisms depending on the relative location between actuation and separation and reduced frequency of actuation (F∗). At an angle of attack of α= 15. 5 ∘, the non-actuated flow separates approximately 4 % chord length downstream of the jet orifices. Plasma synthetic jets (PSJs) applied at F∗≥ 0.5 can displace the separation point downstream to mid-chord position, as a result of the energizing of the incoming boundary layer through mixing enhancement. As a comparison, with actuation frequency of F∗≤ 0.25 , the separation point at α= 15. 5 ∘ remains near the leading edge and the zero-velocity line is periodically swept towards the suction surface by the convecting spanwise vortices generated from PSJ actuation, leading to a reduction of time-averaged backflow area. For the case of separation control at α= 22 ∘, the separation point resides always upstream of the actuation position, regardless of actuation frequency. The peak lift coefficient is attained at F∗= 1 , and the decreasing lift at high actuation frequency (F∗= 2) is ascribed to the severe interaction between adjacent spanwise vortices at short spacing. Graphical abstract: [Figure not available: see fulltext.].