The control of a separated-reattached flow has recently attracted the attention of researchers. The control of a two-dimensional turbulent flow separation from a 25° backward-facing ramp is experimentally studied using an unsteady upstream blowing through a spanwise slit with a view to reducing the recirculation length. The Reynolds number, based on the ramp height, is 9.4 × 10⁴. Three control parameters are examined, i.e. the blowing amplitude, frequency and duty-cycle. The flow is measured extensively with and without control using techniques such as hot-wire anemometry, PIV and pressure scanner. It is found that, given a very small fraction of the input energy, the time-averaged recirculation bubble contracts in length up to 40%, as compared with the natural flow. The optimization control frequency is close to the shedding frequency of the uncontrolled shear layer, suggesting a synchronization between unsteady blowing and vortex shedding. Besides the reduction of separated bubble length, there is an increase of the turbulent activities and a vigorous interaction with the natural vortex shedding.