In order to reduce the sensitivity of hydrogenases in green algae to O₂,four mutants of Chlamydomonas moewusii trains with improved oxygen-tolerance and hydrogen-evolving activities,namely Mu-1,Mu-2,Mu-3 and Mu-4,were generated via ultraviolet mutagenesis and were screened following an exposure to metronidazole and applied oxygen.Then,the screened strains were characterized by the hydrogenases activity in vivo,the respiration activity and the photosystem Ⅱ activity of algal cells.The results show that the hydrogenase activities in vivo of the four mutants are respectively 1.69,2.86,1.06 and 1.80 times that of the wild type,that the residual activities of the mutants after a treatment with 1% O₂（volume fraction） increase respectively from the original（20.00±2.47）% to（34.40±4.88）%,（32.50±2.85）%,（55.60±3.30）% and（43.10±8.73）%,and that,there are no obvious differences in photosynthesis and respiration during the growth of Mu-1,Mu-2 and the wild type.