Abstract: The stable nitrogen isotope and biomass of zooplankton based on size fractionation were studied in the northern South China Sea during summer and winter, 2015. The zooplankton was divided into three groups according to size structure, i.e., micro-zooplankton (180~380 μm), meso-zooplankton (380~500 μm) and macro-zooplankton (>500 μm). The results show that significant spatial difference exist between zooplankton biomass and stable nitrogen isotope. And high values appeared around the Pearl River estuary, Dongsha Islands and south Taiwan Strait. The mean δ¹⁵N value of zooplankton in summer was higher than that in winter, especially that of the micro-zooplankton (P < 0.01), and the δ¹⁵N value of size-fractioned zooplankton generally increased from small to large size zooplankton. The correlation analysis shows that both the δ¹⁵N value of micro-and meso-zooplankton are closely related to zooplankton biomass, while the δ¹⁵N value of macro-zooplankton is not.