This study focuses on the multiple separation-control mechanisms of burst actuation with a dielectric barrier discharge plasma actuator. Control of separated flow around a NACA 0015 airfoil at the Reynolds number 63,000 is investigated with a plasma actuator mounted at 5% chord length from the leading edge. A parametric study on burst frequency and input voltage are conducted on three post-stall angles using time-averaged pressure measurements and time-resolved particle imaging velocimetry (PIV). Trailing edge pressure is chosen for the index of separation control and it indicates that optimum burst frequency is different at each angle of attack. Then, the several flow fields are discussed in detail and the two different flow-control mechanisms are clarified: utilization of large vortex and promotion of turbulent transition. With regard to the first mechanism, the phase-lock PIV indicates that vortex structure, whose size is larger with lower burst frequency in this experimental range, is shed from shear layer for each burst actuation. With regard to the second mechanism, time-averaged pressure and PIV measurements reveal that burst frequency of F+ = 6-10 has a capability for promotion of turbulent transition. Comparing these two mechanisms, only utilizing large vortex structure is effective in higher angle of attack, and, on the other hand, promotion of turbulent transition works better at around the stall angle for separation control.