Reactive oxygen species (ROS) are involved in the oxidative damage of the cyanobacterium Anabaena sp. caused by UV-B (280-315 nm) radiation. UV-B-induced overproduction of ROS as well as the oxidative stress was detected in vivo by using the ROS-sensitive probe 2',7'-dichlorodihydrofluorescein diacetate (DCFH-DA). Thiobarbituric acid reactive substances (TBARS) and fluorometric analysis of DNA unwinding (FADU) methods were adapted to measure lipid peroxidation and DNA strand breaks in Anabaena sp. Moderate UV-B radiation causes an increase of ROS production, enhanced lipid peroxidation and DNA strand breaks, yielding a significantly decreased survival. In contrast, the supplementation of UV-A in our work only showed a significant increase in total ROS levels and DNA strand breaks while no significant effect on lipid peroxidation, chlorophyll bleaching or survival was observed. The presence of ascorbic acid and N-acetyl-L-cysteine (NAC) reversed the oxidative stress and protected the organisms from chlorophyll bleaching and the damage of photosynthetic apparatus induced by UV-B significantly, resulting in a considerably higher survival rate. Ascorbic acid also exhibited a significant protective effect on lipid peroxidation and DNA strand breaks while NAC did not show a substantial effect. These results suggest that ascorbic acid exhibited significantly higher protective efficiency with respect to DNA strand breaks and survival than NAC while NAC appears to be especially effective in defending the photosynthetic apparatus from oxidative damage.