Horizontal distributions of phytoplankton abundance,species composition as well as environmental factors were investigated in the surface waters of south South China Sea (SCS) in early summer (May 16 to June 7) of 2009.In most areas of south SCS,the concentrations of nitrogen and phosphorus were very low and DIN/DIP ratios usually were lower than the Redfield N/P Ratio of 16∶1.Nitrogen nutrients were significant lower in the upwelling region off Vietnam.A total of 144 taxa of phytoplankton were identified in the study area.The dominant genera were Prorocentrum,Gonyaulax,Gyrodinium,Scrippsiella and Chaetoceros,respectively.Spatial patts of early-summer phytoplankton abundance and species composition should be mainly controlled by the upwelling off Vietnam and the discharge of Mekong River in the south SCS.Diatoms dominated in the nutritious Mekong River Estuary or upwelling region off Vietnam; while dinoflagellates dominated in the oligotrophic pelagic region.The canonical correspondence analysis (CCA) indicates that most of diatoms favor higher levels of silicate and phosphorus,as well as lower temperature; while most of dinoflagellates favor the lower silicate and phosphorous and higher temperature.Correlation and CCA results indicate that silicate,nitrate and temperature were the most relevant environmental factors to regulate the horizontal patt of early-summer phytoplankton in the surface waters of south SCS.