We have conducted large-eddy simulations of the turbulent separation control by the DBD plasma actuator over NACA0015 airfoil. Reynolds number based on the chord length is 1,600,000 and the angle of attack is 20.11 degs. At this angle of attack, the flow around the airfoil is massively separated. Effects of a location and operation conditions of the plasma actuator on the separation control are investigated. The most effective location of the actuator to suppress the separation is the vicinity of the turbulent-separation point (2nd separation) and the most effective non-dimensional burst frequency to improve the lift-drag ratio is unity in the burst mode. It is clarified that the effective mechanism for the turbulent-separation control by the burst mode is to induce the pairing of the large-scale vortices near the airfoil surface. This large-scale vortex results in the not only the momentum induction from the freestream to the boundary layer but also the lift improvement by its convection. In addition to this control mechanism, various control effects can be achieved dependent on the settings of the DBD plasma actuator.