Abstract: The distribution of size structure of phytoplankton biomass in surface waters, as measured by chlorophyll a (Chl a), was studied in Qinhou bay during winter and autumn in 2014. In winter and autumn, average total Chl a concentration was 2.38μg/L and 6.96μg/L, respectively. Phytoplankton biomass was dominated by nano-phytoplankton in winter, accounting for 73.0% of the total Chl a concentration, followed by micro-phytoplankton with 14.9% and pico-phytoplankton with 12.1%. In autumn, micro-, nano-and pico-phytoplankton contributing to 42.3%, 44.7% and 12.9% of the total Chl a concentration respectively. The average size index of phytoplankton was higher in autumn (23.55μm) than in winter (11.23μm). As a general trend, lager phytoplankton (>20μm) contributed significantly to the biomass in nutrient-repleted waters, and the relative proportions ofnano-and pico-phytoplankton increased from the north to the south in the study area.Temperature, nutrients and suspended solids seems to be the main factors controlling the distribution and variation of size-fractioned Chl a in the surface waters of Qinzhou bay.