The purpose of this study is to set up purification procedure of Cangshan garlic oligosaccharide(GOS) and determination its molecular weight.The purification effects of polyamide chromatography,DEAE-cellulose (OH~- ) chromatography,ethanol precipitation method and Sephadex G-25 chromatography on GOS was explored,respectively. The molecular weight of GOS was determined by HPLC method.The results showed that polyamide column chromatography could remove 66.8%of the protein and the GOS recovery ratio was 98.9%.DEAE-cellulose(OH~- ) chromatography separated GOS into 3 groups.Ethanol precipitation method separated GOS into 8 groups.After purified by Sephadex G-25,the purification of GOS reached to 99.1%.The Mw/Mn was 1.31 and Mn was 1770.