Abstract: Hyphenated GC and MS was used in the determination of acetylation products of 5 common monosaccharides,i.e.,xylose,arabinose,mannose,glucose and galactose.For obtaining high efficiency of pre-derivatization,conditions for acetylation,including the time and temperature of reduction,time and temperature of acetylation and etc.,were studied and optimized by using the orthogonal experimental designing method.It was found that the temperature of reduction and time of acetylation were found to be the significant factors influential to the recovery of monosaccharides.The optimized acetylation conditions were found as follows:① temperature of reduction:50 ℃;② time of reduction:2.5 h;③ temperature of acetylation:90 ℃ and ④ time of acetylation 2.5 h.