A time-periodic blowing/suction is provided to control turbulent separation in a boundary layer using direct numerical simulation. The blowing/suction is given just before the separation point, and its nondimensional forcing frequency ranges from F* = fLb/U∞ = 0.28–8.75, where f is the forcing frequency, Lb is the streamwise length of uncontrolled separation bubble, and U∞ is the freestream velocity. The size of separation bubble is minimum at F* = 0.5. At low forcing frequencies of F* ≤ 0.5, vortices generated by the forcing travel downstream at convection velocity of 0.32–0.35 U∞, bring high momentum toward the wall, and reduce the size of separation bubble. However, at high forcing frequencies of F* ≥ 1.56, flow separation disappears and appears in time during the forcing period. This phenomenon occurs due to high wall-pressure gradients alternating favorably and adversely in time. A potential flow theory indicates that this rapid change of the wall pressure in time occurs through an inviscid mechanism. Finally, it is shown that this high-frequency forcing requires a large control input power due to high pressure work.