We investigated the abundance and species composition of phytoplankton community in the northern South China Sea (110°－120°E, 18°－23°N) from 10th to 29th August 2007, using 208 samples taken from 41 sampling stations. A total of 216 species belonging to 80 genera of 5 classes were identified. Diatoms contributed to 64.81% of the total phytoplankton species number and 89.11% of the total phytoplankton cell abundance; followed by dinoflagellates, which accounted for 30.56% of total phytoplankton species number and 10.62% of the total phytoplankton cell abundance. The dominant species were Thalassiosira sp., Thalassionema nitzschioides, Skeletonema costatum, Prorocentrum minimum and Gymnodinium sp.. Trichodesmium erythraeum that belonged to Cyanophyta was also commonly found at many survey stations. High phyto-plankton cell abundance was located in the Pearl River Estuary and in Taiwan Shoal. The horizontal distribution pattern of phytoplankton abundance was different in the surface and sub-surface waters. In the surface water, the average abundance of phytoplankton was 25.21×103cells•L?1 with a high value near the Pear River Estuary; while in the sub-surface water, the aver-age abundance was 9×103cells•L?1 with a high value in the Taiwan Shoal. On the whole, phytoplankton abundance was lower at deep-water stations (>200m) than at shallow-water stations (<200m). In general, the vertical profiles showed that total phytoplankton abundance presented a peak at 10m and then decreased with increasing water depth. The vertical distribution patterns of phytoplankton abundance also appeared to be different between deep-water and shallow-water stations. Diatoms abundance decreased with increasing water depth at the shallow-water stations, but formed a maximum peak at sub-surface (75m) of the deep-water stations; dinoflagellates abundance significantly decreased with increasing water depth, but evenly distributed in the water above 75m at the deep-water stations. The ratios of diatom to dinoflagellates showed a decline ten-dency approaching to the pelagic region. Higher productivity and phytoplankton abundance regions corresponded with a larger ratio of diatom to dinoflagellates. At the survey stations, the average Shannon-Wiener diversity index of phytoplankton com-munity was 2.8 in the surface water and 3.0 in the sub-surface water. High phytoplankton biodiversity located in the Pearl River Estuary and the coasts of Guangdong and Hainan provinces. The diversity of phytoplankton at the shallow stations was far enriched than that at the deep-water stations.