The performance and mechanism of an acoustic feedback control applied to flow oscillation in a jet-wedge system were investigated experimentally. The self-sustained oscillations were effectively suppressed by imposing velocity fluctuations near the nozzle exit, which were fed by the fluctuating pressure signal at the wedge with a certain phase lag and feedback gain. The optimum condition for the imposed velocity amplitude was found to be as small as 1.2% of the jet velocity. The variation of the flow field with and without control was studied by flow visualization with high-speed camera and by flow-field measurements with particle image velocimetry. These observations indicate that the primary mode of the jet oscillation in the jet-wedge system is weakened by the feedback control to a level of secondary mode. This is mainly due to the destruction of the synchronized flow structure of the jet-wedge system by the active control, which is found in the mean and fluctuating velocity distribution and the velocity correlation over the wedge and along the jet shear-layers.