Flows over a square cylinder with and without plasma actuation are numerically investigated to see whether plasma actuation can effectively modify vortex shedding from the cylinder and reduce the drag and lift fluctuations. In this study, a plasma synthetic jet actuator is mounted on the rear side of cylinder as a means of direct-wake control. The effect of plasma actuation is considered by adding a momentum forcing term in the Navier-Stokes equations. Results show that the reduction of mean drag and lift fluctuations is obtained for both steady and unsteady actuation. However, the steady actuation is better than the unsteady one in terms of mean drag as well as drag fluctuations. With the strong steady actuation considered, the interaction of two separating shear layers from rear corners is effectively weakened due to the interference of synthetic jets. It results in a merging of synthetic-jet and shear-layer vortices and the increase of vortex shedding frequency. On the other hand, the unsteady actuation generates pulsating synthetic jets in the near wake, but it does not change the vortex shedding frequency for the actuation frequencies considered in this study.