Biomass size spectrum (BSS) is constructed by allocating all organisms into size classes according to their individual body mass (size) and computing the total biomass per size class. It proves to be an effective ataxonomic approach for analyzing the structure of the pelagic ecosystem. However, empirical data regarding planktonic size-structure in adjacent waters of Yangtze River Estuary are scarce. Based on the data from survey carried out in spring, 2005, the individual size compositions and BSS of the net plankton were explored in the in adjacent waters of Yangtze River Estuary. Plankton samples were collected vertically from 31 stations with micro-, meso- and macro- plankton nets (77, 160μm and 505μm mesh size), respectively. Results revealed an approximately continuous size distribution of plankton individuals, from phytoplankton (5-250μm cell -1 in equivalent sphere diameter (ESD), 15pg-146ng C cell -1) to zooplankton (120μm-2cm ESD, 115ng-7.5mg C ind-1). The distribution of normalized biomass versus size (carbon scale) was linear on a double log plot at all stations. The normalized spectra slopes of the study area ranged from -0.889 to -0.445, and 12.866 to 16.863 (all stations together, slope=-0.606, intercept=19.448), indicating strong deviations from the ideal value (slope =-1.22) of a steady pelagic ecosystem. The slopes and intercepts varied significantly among the stations (p<0.0001) and the higher values occurred in the southern part of study area. It concludes that the intercepts and correlation coefficients of net plankton spectra covaried roughly with size diversity and evenness in total plankton composition.