An oscillatory, zero-net-mass flux actuator system, Jet and Vortex Actuator (JaVA), is implemented on the step wall of a backward facing step (BFS) in order to control the recirculation zone behind BFS. The actuator plate has a sinusoidal motion determined by the amplitude and the operating frequency. The proposed control method based on suction and blowing with an oscillating backward facing step seems to be effective in reducing the recirculation zone length and preventing flow separation downstream of BFS. Modal investigation of flow fields are performed by means of Proper Orthogonal Decomposition (POD). POD is applied to time-dependant snapshots of velocity fields at several operating frequencies. The POD modes revealed the behavior of the flow past a BFS under the effect of active flow control.