Abstract: In this paper,EI and FAB-MS of tri and tetra-O-acetyl C-glucosides and C-galactosides were reported. Some characteristic fragmentation patterns of O-acetyl C-glycosides were discussed. In result,FAB-MS can give the clear protonated molecular ion peaks(M+H~+) and some fragment peaks formed after losssing water and acetic acids which is very useful in the determination of the molecular weight. EI-MS usually provides more informations about the structrure of the compounds.