In this paper, the mechanisms of control of separated flow around NACA0015 of angle of attach 12 degree at Reynolds number 63,000 using plasma actuator are classified and discussed. A series of large-eddy simulations using compact scheme is conducted, and results are discussed. Especially, the flow control mechanism related to the separation bubble is discussed for the cases with the burst actuation of plasma actuator at nondimensinal burst wave frequency of 1 and 6 based on chord length and freestream. The averaged flow fields show that the case with the nondimensional burst wave frequency of 6 has earlier and smooth transition and it uses the turbulent mixing effectively. This earlier transition is because the actuation with the nondimensional burst wave frequency of 6 effectively excites the Kelvin-Helmholz instability. On the other hand, though the phase averaged flow fields illustrate that the case with nondimensional frequency of 1 uses the mixing by the large vortex more than F+ = 6, the periodic components of Reynolds stress is much smaller than turbulent components of that. This result show that, at least in terms of Reynolds stress, the turbulent mixing is more important for flow control in this situation.