The flow structures in a separated shear layer actuated by a synthetic jet actuator were studied using experimental methods. When forced at a frequency much lower than the natural shedding frequency (fH/U=0.042 or fXr/U=0.24), the vertical flapping motion of the shear layer downstream of the separation point became prominent. The size of the peak in the pressure spectra at the forcing frequency (StA=fAH/U) measured near the separation point (x/H=1) increased linearly with the forcing amplitude (u′/U) suggesting a linear response of the pressure fluctuations to the forcing by the synthetic jet. The linear response did not hold for the pressure fluctuations away from the jet exit as the magnitude of the peak for StA measured at x/H=3 soon saturated when the forcing amplitude became larger than 0.3.