Abstract: Objective: To analysis the difference of monosaccharide compositions and their antioxidant activity of polysaccharides which were sank by various concentrations of ethanol.Methods: The monosaccharide composition of Noni polysaccharides, which were extracted with ultrasonic-assisted method and precipitated with increasing ethanol concentration, were decolorized with macroporous resin and deproteinized with Sevage method before being determined by precolumn derivatization HPLC. What's more, the differences of their antioxidant activity were also compared with three kinds of antioxidant activity evaluation methods.Results: Four polysaccharides samples all mainly composed with mannose, rhamnose, glucose, galactose and arabinose, only the mole percentages of four polysaccharides samples were somewhat different. The monosaccharide mole percentages of four polysaccharides samples sank by the concentration of 30% ( A) , 50% ( B) , 70% ( C) , 90% ( D) ethanol were 1.33 ∶ 4.24 ∶77.22∶ 9.83 ∶ 7.38, 2.58 ∶ 2.28 ∶ 52.20 ∶ 31.40 ∶ 11.54, 1.32 ∶ 6.45 ∶ 47.00 ∶ 32.91 ∶ 12.31, 3.18 ∶ 3.39 ∶ 27.23 ∶ 45.50 ∶ 20.70. The scavenging ability of DPPH· and O₂^-·of four polysaccharides samples were in the same order of D > C > B > A, and the order of scavenging ability for OH·was C > B > A > D. Conclusion: The monosaccharide compositions of four parts of polysaccharides were same, but the mole percentages were different, the antioxidant activity of polysaccharide sank by 70% concentrations ethanol was the highest.