An active dimple with oscillating bottom surface is developed for flow separation control of low Reynolds number diffuser flow. This device can provide a periodic disturbance into the flow by means of an amplified piezoelectric actuator. In the present study, basic characteristics of the active dimple were obtained by measuring an induced flow around the dimple using dynamic PIV. The influences on separated flow were also evaluated based on the pressure recovery in a diffuser. The dimple dimension was 5 mm in diameter and 0.1 〜 0.5 mm in amplitude. Reynolds number of the main flow based on the channel height is 650. As the oscillating amplitude increases, the induced flow velocity is changed and the flow structure as well. The strength of the induced flow has a great effect on the pressure recovery at the diffuser.