The radical scavenging activities of polysaccharides in three Nostoc species were investigated employing three in vitro systems. The result showed that they all have a concentration-dependent scavenging effect on superoxide anion radical and hydroxyl radical. N. commune,with a high inhibition rate of 72.3% at 40 mu g/mL, was the most effective scavenger on superoxide radical, while that were 46.7% and 35.5% of N. sphaeroides and N. flagelliforme respectively. Moreover, N. flagelliforme showed an excellent capacity on hydroxyl radical scavenging. Its maximal scavenging effect was 74.3% at the test concentration of 30 mu g/mL, while 49.0% and 46.7% were N. sphaeroides and N. commune's, respectively. However, three Nostoc polysaccharides had no significant effect on DPPH radical scavenging.