Daya Bay is one of a series of large bays along the southern coast of China, characterized by low rates of water exchange with seawater. Daya Bay has been subjected to red tides, which have attributed to a deterioration of ecosystem over several decades. To assess its current status, phytoplankton composition, abundance, community structure and diversity were investigated in each season in 2002. Furthermore, the relationships were discussed between its characteristics and the variation of environmental factors, mainly nutrients, temperature influenced by anthropological activity and salinity. Results showed that a total of 48 genera and 114 species of phytoplankton were identified. The largest component was Bacillariophyta containing 84 taxa. Pyrrophyta was the next most common with 9 genera and 23 species. Cyanophyceae and Chrysophyta were also recorded within the study area. The cells abundance of phytoplankton varied from 5.79×10⁴to 5.37×106 cells/m^３ with an average of 1.14×10⁶ cells/m^３.Diatoms (the average of 1.08×10⁶cells/m^３) were the predominant abundance recorded over the year. However, the percentage of diatoms (75.8%) in autumn was lower than other seasons. Dinoflagellates (the average of 9.91×10⁴cells/m^３) were the secondly dominant group. Annual variatio-ns of the phytoplankton community in the Daya Bay show a typical one-peak cycle, with the highest cell abundance peak recorded in summer and the lowest recorded in autumn. The ecotypes of phytoplankton were mostly alongshore warm-water species, however, marine warm-water species and eurytopic species in winter and autumn are more abundant than the other seasons, especially in winter, which were dominant ecotypes. In Daya Bay the dominant species were diverse and variable with seasons. Compared with the data in the last two decades, some dominant species, the dominant sequence and cells abundance have changed. Moreover, the total species decrease, but big cells diatoms appearing mainly in winter and spring have increased since 1995, which has attributed toward a higher primary production over this time period. The species diversity index of phytoplankton in Daya Bay was low in summer, and the species diversity index especially around the nuclear power station and aquafarm was low in summer and autumn. Community structure and cells abundance were separated in relation to monsoon, current and anthropological activities. It is presented that the temperature and hydrodynamics in conjunction with the pattern of nutrients (DIN, DIP and the N/P) availability and depletion affect the phytoplankton composition, abundance, community structure, community succession and the diversity.