The molecular weights,chemical compositions and scavenging activities on oxygen free radicals of various sulfated polysaccharides and alginate from Laminaria japonica were compared to make clear the structureactivity relationship and mechanism of antioxidant activities of polysaccharides on oxygen free radicals.The results showed that low sulfated fraction F-A2,with a peakmolecular weight(Mp)of 6-15 ku exhibited a very strong antioxidant activity on superoxide and hydroxyl radicals,however,the highly sulfated fractions with Mp below 15 ku had vere low antioxidant activity.The low molecular weight alginate especially fraction with molecular weight below 1 ku obtained by enzymatic hydrolysis had higher scavenging activity than sulfated polysaccharides from L.japonica.These results indicated sulfate group of low molecular weight sulfated polysaccharide seems a dimensional block for the reaction with oxygen radicals,while glucuronic acid content contributes much to the scavenging activities of polysaccharides on oxygen free radicals.The chemical properties and scavenging activities of sulfated polysaccharide fractions Fa1(Mp 8 ku)and Fa2(1 ku)obtained by hydroxyl radical process degradation of crude sulfated polysaccharide showed that hydroxyl radical process mainly degraded the ascopyllan-like species rich in glucuronic acid,mannose and glucose,while fraction rich in sulfated L-fucose remained unchanged.These results suggest that antioxidant activity of sulfated polysaccharides is apparently related not only to molecular weight and sulfated ester content,as previously determined,but also to the glucuronic acid,fucose contents and neutral sugars in polysaccharides chain.