The flowfield induced by a plasma synthetic jet actuator was investigated by solving Reynolds-averaged Navier- Stokes equations, augmented with a phenomenological model representing the plasma-induced body force imparted by a plasma actuator on the fluid. Flow characteristics of an unsteady plasma synthetic jet in quiescent air were presented. The results indicate that the time-averaged flowfield is quite similar to the steady plasma synthetic jet, the traditional blowing synthetic jet, and the plane jet. The time-averaged streamwise velocity is a little smaller than that of the steady case, but the jet half-width is much larger. This makes the volume and momentum flux of an unsteady plasma synthetic jet larger. With unsteady actuation, a plasma synthetic jet does not exhibit the suction effect of the cavity during the second half-period, as does the traditional blowing synthetic jet. The vortex pair, formed from the wall jets that are produced by plasma actuators, has a strong unsteady effect on the plasma synthetic jet close to the wall. In the far field, the vortex pair coalesces and eventually develops to a continuous jet.