A massive number of large-eddy simulations of the separated flow over NACA0015 airfoil, which are controlled by a DBD plasma actuator, are conducted. Reynolds number based on chord length is 63,000 and the angle of attack is from 12 to 18, which are stall angle in present flow condition. The position and operation conditions of a DBD plasma actuator, (e.g. the burst frequency, the degree of induced flow and the burst ratio of actuation) are varied as parameters. It is clarified that the most effective position of the actuator to suppress the separation is vicinity of the separation point. The most effective burst frequency of burst wave to improve the lift-drag ratio is F+ ≈ 5. In the cases of these optimal position and burst frequency, the energy consumption by actuation can be reduced so much. The promotion of turbulent transition is closely related to the control of separation. The simple analyses of turbulent kinetic energy distributions clarify that the cases with quick turbulent transition over airfoil have better aerodynamic performance. In addition, other mechanisms of the separation control are also shown for each angle of attack, and the effect of control are classified in terms of the improvement of aerodynamic performances.