The objective of this study is to control the reattachment process of the separating and reattaching flow over a backward facing step in a low Reynolds number range by the synthetic jet. The Reynolds number based on the step height ranges from 300 to 900. The synthetic jet has a frequency of 10, 36, 70, and 150Hz. In order to investigate the effect of the periodic disturbance on the reattachment process, Micro Flow Sensor was used for measurement of phase-averaged forward flow fraction. The results show effectiveness of the synthetic jet for flow control. The reattachment process strongly depends on Reynolds number and a vortex motion which is generated at the separating shear layer.