A large-scale cyanobacterial bloom was observed for the first time in Xiangxi Bay in summer 2008, which lasted from early June to late July dominated by the species of Anabaena sp. Microcystis aeruginosa and Microcystis wesenbergii. The cell abundance and relative abundance is up to 3.82×10⁸ cells/L and 90%, respectively, during the peak. Controlled by nutrients, temperature and water column stability, cyanobacterial bloom in the TGR was most likely to occur in June and July. In the May of 2008, the ratio of N:P was 6.2, while 18.6 in the May of 2007. As reported, the lower N:P ratio (< 8) in the bay is beneficial to the recruitment of cyanobacteria, providing a seed resource for Microcystis to proliferate under appropriate conditions. In addition, the solar irradiance from May to July, 2008, was significantly higher than that in 2007, which may be one of the causes of cyanobacterial bloom in 2008. We supposed that on the basis of adequate nutrients, elevated water temperature and high water column stability, the low N:P ratio and high irradiance was inducible for the Microcystis recruitment, which maybe the major factors that brought about the cyanobacterial bloom.