Abstract: In this paper, the acoustic characteristic of microphone slit-probe in high speed flow, with Mach number up to 0.8, was investigated experimentally. Results show that the acoustic flow resistance of the cover-cloth of the slit-probe has a significant influence to the acoustic characteristic of the probe,and that the frequency response of acoustic sensitivity can be smoothed by inserting a additional pipe filled with absorbent material inside the probe. Experiment also indicates that the static and dynamic calibraions of probe are important to the correction of the acoustic measurment in high speed flow.