Abstract: Influences of nutrients and illuminance on phytoplankton community and its dominant species were studied at the net cage aquaculture area in the baymouth of Shacheng Harbour in April, 2006. The main results were as follows: micro-nanophytoplankton got best growth under 100% natural illumination, while picophytoplankton got best growth under 50% natural illumination. Ammonia nitrogen was first utilized at the presence of different forms of nutrients, but afterwards the phytoplankton abundance in ammonia-nitrogen enrichment group was lower than that of nitrite and nitrate enrichment groups. Nutrients enrichment caused rapid growth of phytoplankton in a short time, but it inhibited phytoplankton growth when it exceeded a certain level and the inhibition was associated with the phytoplankton species. Phytoplankton abundance and biodiversity decreased as nutrients were consumed. Micro-nanophytoplankton abundance was significantly correlated with nitrate and nitrogen-phosphorus ratio, with correlation coefficients of －0.625 and 0.521 respectively. Picophytoplankton was significantly correlated with phosphate, with correlation coefficient of 0.530.