Population changes in Chaetoceors spp. and environmental factors were investigated in Daya Bay, South China Sea, from June 1997 to July 1998 and the spring of 1999 and 2000. Relationships between abundance of Chaeto. spp. and some environment parameters were analyzed by multivariate general linear factorial analysis, linear stepwise regression analysis, and principle component analysis. Chaetoceros was the most abundant genus of phytoplankton in Daya Bay, contributing 23.25% of the total phytoplankton abundance. Chaetoceros reached peak abundance in the spring of 1998, with maximum cell density of 6680.6 cells/ml and 99.78% of total phytoplankton. C.affinis, C.debilis, C.distans and C.lorenzianus were the main bloom causative species. The nutrient levels were high just before the bloom of Chaetoceros, but dropped rapidly as the bloom developed. Results showed that depletion of nutrients especially dissolved inorganic nitrogen (DIN) led to the collapse of Chaetoceros bloom. Peak abundances of Cheatoceros occurred under a wide variety of water temperature (15℃ to 31℃) and salinity (15psu to 31psu). Results from statistical analyses revealed that water temperature and ratios between nutrients were the key factors influencing on population dynamics of Chaetoceros, while DIN and salinity played important roles as well.