Abstract: In this study, phytoplankton community was investigated and analyzed fromJul. 1997 to Jun. 1998 in Daya Bay. A total of 198 phytoplankton species, belonging to 65 genera was recorded and in terms of species composition and abundance, diatoms dominated throughout the whole year. Dominant species of diatoms wereChaetoceros spp.,Skeletonema costatum,Pseudo-nitzschia spp. whereas dominant species of dinoflagellates wereGymnodinium spp.,Prorocentrum spp.. Phytoplankton abundance showed significant seasonal change and diatoms had two peaks in spring and autumn with dinoflagellates only one peak in spring. Horizontal distribution of phytoplankton abundance showed no obvious difference among the six stations investigated,but the cell density of S2 which near to the cage2fish cultural area was slightly less than the others. The relationships between major phytoplankton species(including three dominant diatoms and three dominant dinoflagellates) and water temperature, salinity wre analyzed respectively. The results showed that temperature and salinity had no limitary effect on the appearance of diatoms, meanwhile dinoflagellates appeared during 17-30℃, with optimal in 25℃. But seasonal change of temperature affected obviously on dominant speciespgrowth and depletion. The annual variation of diversity indexes(D,H＇) and evenness (E) of phytoplankton showed no clear seasonal and spatial pattern and the average values of them were 0.611,2.107,0.557 respectively. Compared with other bays in Southeast China Sea, the diversity indexes and evenness are commonly high except in early spring.