Abstract: Based on size fractionation zooplankton (>500 μm, 380-500 μm and 180-380 μm) samples collected in the western South China Sea in 2018 summer, biomass, δ¹³C and δ¹⁵N were studied. The results show that the biomass of zooplankton ranged from 0.25 mg/m³ to 8.44 mg/m³, the δ¹³C of zooplankton ranged from −26.73‰ to −14.84‰, and the δ¹⁵N of zooplankton ranged from 3.44‰ to 8.40‰. Due to the influence of coastal upwelling and cyclonic eddy, high values of zooplankton biomass and stable isotope appeared around the central and western in study area. The stable isotopes of carbon and nitrogen of zooplankton showed significant differences in particle size structure. The nitrogen stable isotopes of size-fractioned zooplankton generally increased from small to large size zooplankton, and the δ¹³C of meso-zooplankton were significantly lower than micro-zooplankton and macro-zooplankton. The δ¹³C and δ¹⁵N of size-fractioned zooplankton showed similar spatial distribution characteristic. And high values appeared around the central and western of the study area. The correlation analysis shows that stable nitrogen isotope of micro-zooplankton and macro-zooplankton were positively correlated with zooplankton biomass, while the stable carbon and nitrogen isotope of meso-zooplankton were not. This study provides basic materials for the structure of marine food webs in the western South China Sea.