Abstract: Total chlorophyll a and size structure of phytoplankton community were investigated in Shenzhen bay, a highly eutrophic bay in the southern China.From November 2010 to August 2011, total chlorophyll a was high with an annual mean of 10.95 mg/m³ and a range from 0.63 mg/m³ to 186.99 mg/m³.The highest concentration of total chlorophyll a was found in autumn (25.45 mg/m³), followed by summer (11.18 mg/m³), spring (4.95 mg/m³) and winter (2.21 mg/m³).In all the four seasons, the concentration of chlorophyll a showed a similar spatial pattern, higher in coastal areas and lower in open areas.Phytoplankton was dominated by nano-sized species in spring and summer (>74%) and by micro-and nano-sized species (>84%) in autumn and winter.Pico-phytoplankton had very low contribution to total chlorophyll a all year around ( < 16%).By Pearson's Correlation analysis, micro-chlorophyll a had a significant positive correlation with dissolved inorganic nitrogen (p < 0.05); and nano-fraction had a significant positive correlation with dissolved inorganic nitrogen, dissolved inorganic phosphorus, and silicate (p < 0.05).These results suggested that the high concentration of total chlorophyll a and the dominance of micro-and nano-phytoplankton species was mainly due to high concentrations of nutrients.Light availability and temperature were also important in spring and winter.