Recent evidence has shown active N2fixation in coastal eutrophic waters, yet the rate and controlling factors remain poorly understood, particularly in large estuaries. The Changjiang Estuary (CE) and adjacent shelf are characterized by fresh, nitrogen-replete Changjiang Diluted Water (CDW) and saline, nitrogen-depletion intruded Kuroshio water (Taiwan Warm Current and nearshore Kuroshio Branch Current), where N2fixation may be contributed by different groups (i.e.,Trichodesmiumand heterotrophic diazotrophs). Here, for the first time, we provide direct measurement of size-fractionated N2fixation rates (NFRs) off the CE during summer 2014 using the15N2bubble tracer method. The results demonstrated considerable spatial variations (southern > northern; offshore > inshore) in surface and depth-integrated NFRs, averaging 0.83 nmol N L−1d−1and 24.3 μmol N m−2d−1, respectively. The highest bulk NFR (99.9 μmol N m−2d−1; mostly contributed by >10 μm fraction) occurred in the southeastern East China Sea, where suffered from strong intrusion of the Kuroshio water characterized by low N/P ratio (<10) and abundantTrichodesmium(up to 10.23 × 106trichomes m−2). However, low NFR (mostly contributed by <10 μm fraction) was detected in the CE controlled by the CDW, where NOxconcentration (up to 80 μmol L−1) and N/P ratio (>100) were high andTrichodesmiumabundance was low. The >10 μm fraction accounted for 60% of depth-integrated bulk NFR over the CE and adjacent shelf. We speculated that the present NFR of >10 μm fraction was mostly supported byTrichodesmium. Spearman rank correlation indicated that the NFR was significantly positively correlated withTrichodesmiumabundance, salinity, temperature and Secchi depth, but was negatively with turbidity, N/P ratio, NOx, and chlorophyllaconcentration. Our study suggests that distribution and size structure of N2fixation off the CE are largely regulated by water mass (intruded Kuroshio water and CDW) movement and associated diazotrophs (particularlyTrichodesmium) and nutrient conditions.