Abstract: Four cruise surveys were conducted on the phytoplankton in Qinzhou Bay of South China in different seasons in 2008-2009. A total of 131 phytoplankton species were identified, among which, diatom (101 species) accounted for 30.0% of the total, followed by pyrrophyta (23 species), and other groups (7 species). Most of the phytoplankton was of eurytherm or warmwater species. The total species number and the diatom abundance were the smallest in spring, and increased successively in summer, autumn, and winter, being the highest in winter. The phytoplankton density varied from 232.28×10⁴ cell·m^-3 to 977.0×10⁴ cell·m^-3, with an average of about 558.57×10⁴ cell·m^-3. Temporally, the phytoplankton density was the highest in summer, followed by in spring, and the lowest in winter and autumn. Spatially, the phytoplankton density was increased from the inner bay to the open bay, and decreased from the open bay to outside the bay. In summer, the highest density area switched from the open bay to the bay’s entrance. The average Shannon diversity index was 3.18, and the average evenness index was 0.63, suggesting a high diversity of the phytoplankton community. The correlations of the phytoplankton density with the water temperature, salinity, dissolved inorganic phosphorus, and dissolved inorganic phosphorus  varied with seasons.