Abstract: In this paper we found that:1.The nitrogenase activity of Anabaena 7120decreased and was more sensitive to oxygen,when treated at high temperaturefor a short time. 2.The O2-sensitivity of nitrogenase activity of Anabaena drivenby stress of high temperature was increased with the increase of O2-concentrationin test system.3. Molecular hydrogen could protect nitrogenase activity from damageby oxygen in Anabaena injured by high temperature.4.The protection of nitroge-nase activity by molecular hydrogen against O2 inactivation in Anabaena influencedby high temperature was more obvious than that in normal algae cells.5.Theprotection of nitrogenase activity by molecular hydrogen against O2-inactivationin Anabaena influenced by high temperature was occured when the photosyntheticinhibitors,such as DNP or CCCP,or CO2 or N2 were presented respectively, butnot when external sucrose presnted.6.When algae cells injured by high temper-ature were preincubated in the dark in 60-70%C2H2 and 30-40% Ar atmosphereor in 10% CO the protective action of hydrogen was weakened or disappeared.