The present invention provides a process for preparing orlistat (I) by alkanoylating an amino orlistat using formic acid anhydride as an alkanoylating agent to obtain orlistat substantially free of the byproduct, (S)-N-acetylleucine (1S)-1-[[(2S, 3S)-3-hexyl-4-oxo-2-oxetanyl]methyl] dodecyl ester (deformyl-N-acetyl orlistat). ##STR00001##