To study the role of nitric oxide ( NO) on enhanced ultraviolet-B ( UV-B) radiation ( 280 - 320 nm)- induced damage of Cyanobacterium, the growth, pigment content, and antioxidative activity of Spirulina platensis 794 cells were investigated under enhanced UV-B radiation and under different chemical treatments with or without UV-B radiation for 6 h. The changes in chlorophyll-a, malondialdehyde content, and biomass confirmed that 0.5 mM sodium nitroprusside ( SNP), a donor of nitric oxide ( NO), could markedly alleviate the damage caused by enhanced UV-B. Specifically, the biomass and the chlorophyll-a content in S. platensis-794 cells decreased 40% and 42%, respectively under enhanced UV- B stress alone, but they only decreased 10% and 18% in the cells treated with UV- B irradiation and 0.5 mM SNP. Further experiments suggested that NO treatment significantly increased the activities of superoxide dismutase ( SOD) and catalase ( CAT), and decreased the accumulation of O-2(-) in enhanced UV-B-irradiated cells. SOD and CAT activity increased 0.95- and 6.73-fold, respectively. The accumulation of reduced glutathione ( GSH) increased during treatment with 0.5 mM SNP in normal S. platensis cells, but SNP treatment could inhibit the increase of GSH in enhanced UV- B-stressed S. platensis cells. Thus, these results suggest that NO can strongly alleviate oxidative damage caused by UV- B stress by increasing the activities of SOD, peroxidase, CAT, and the accumulation of GSH, and by eliminating O-2(-) in S. platensis-794 cells. In addition, the difference of NO origin between plants and cyanobacteria are discussed.