Abstract: There is a heat stable oxygen-scavenging system (OSS) associated with membrane which reduces oxygen endogenously in cells of blue-green algae. Addition of the OSS to cell suspension of heterocystous oxygen sensitive Anabaena mutant and non-heterocystous Pleetonema boryanum led to an increase in their nitrogenase activity by 10–100-fold higher than those under microaerobic condition and also could restore effectively their acetylene reduction activity at higher oxygen concentration since the oxygen presented was reduced effectively. The results suggest that the OSS possesses a function protecting nitrogenase from oxygen in cells. Furthermore, it was found that the efficiency of reducing oxygen of OSS from the Anabaena mutant and Plectonema was lower than those from Anabaena wild and Gloeocapsa in atm. oxygen level. This may be ralated with the susceptibility of nitrogen fixation to oxygen in the cells of Anabaena mutant and Plectonema. The present study firstly indicades the relationship between the heat stable OSS associated with membrane and the mechanism of protecting nitrogenase from oxygen in cells of blue-green algae. Activities of catalase, peroxidase and superoxide dismutase do not show obvious difference in cellfree extract of Anabaena wild and mutant. Methyl viologen can induce nitrogenase activity of Anabaena mutant by subverting a portion of electon flow to accelerate oxygen reduction.