Large-eddy simulations are conducted to investigate the control effects of a dielectric barrier discharge plasma actuator on a dynamic flowfield with a Reynolds number of 2.56 × 105 and a reduced frequency of 0.02π. The objective flowfields include dynamic stall phenomenon, flow separation and reattachment. First the flowfield without control is investigated and it is found that dynamic stall process can be classified into five stages; formation of a laminar separation bubble, breakdown of the laminar separation bubble which triggers formation of a dynamic stall vortex, convection of the dynamic stall vortex, full stall from the leading edge, and recovery to the attached state. Then the control effects with three burst frequencies (F +) of 0.5, 6, and 50 in nondimensionalized value are investigated. The DBD plasma actuator successfully enhances the cycle-integrated aerodynamic performances of the airfoil and major control effects are summarized into three; delay of dynamic stall, enhancement of aerodynamic forces during full stall by large vortices, and promotion of reattachment. The most effective burst frequency for each control effect differs from each other, showing that the best case for delaying the dynamic stall onset is the case with F + of 50 while the best case for promoting the reattachment is the case with F + of 6. The results show that the promoting the reattachment is effective for improving the cycle-integrated net damping and shortening the duration under the stall. For further improvement, the current results give a strong prospect of a closed-loop control in which F + is adapted to the change in the flowfield.