The Alpha Magnetic Spectrometer (AMS) was flown in june 1998 on board of thespace shuttle Discovery (flight STS-91) at an altitude ranging between 320 and390 km. This preliminary version of AMS included an Aerogel Threshold Cherenkovdetector (ATC) to separate antiprotons from electron background, for momentaless than 3.5 GeV/c. In this paper, the design and physical principles of ATCwill be discussed briefly, then the performance results of the ATC will bepresented.