Small jellyfish (here including medusae, siphonophores, and ctenophores) are widespread in the marine pelagic realm. They usually constitute one of the most important assemblages of predators in pelagic food webs by serving as a direct link between zooplankton and higher trophic levels. They can at times be the most abundant non-crustacean invertebrate predators due to their great voracity and their rapid population growth, resulting in extensive seasonal blooms. Thus, studies on small jellyfish have become popular in marine ecology. Studies concerning zooplankton distribution at large spatial scales, particularly for weak swimmers like small jellyfish, emphasize the importance of the physical characteristics of water masses, including salinity and temperature. In our study, the seasonal abundance and horizontal distribution of small jellyfish were assessed in the Changjiang Estuary (Yangtze River) and the adjacent East China Sea (29°00'-32°00'N, 122°00'-123°30'E) in 2002-2003. The Changjiang Estuary and the adjacent East China Sea are one of the most productive aquatic ecosystems. High primary production supports high fishery production in adjacent coastal waters, including the Changjiang and Zhoushan fishery grounds. A total of 41 species of small jellyfish (11 siphonophores, 28 medusae, 2 ctenophores) were collected. Numerically, siphonophores were most abundant (58.56%-91.05% of the total collection by number). The dominant species were the siphonophores Diphyes chamissonis, Muggiaea atlantica, and Physophora hydrostatica and medusae Liriope tetraphylla, Solmundella bitentaculata, and Eirene brevistylus. Muggiaea atlantica was dominant all year round, particularly in spring (76.19% of the total collection by number) and winter (39.12%). D. chamissonis was dominant in summer and autumn, with abundances of 249.03 ind./m² (73.82% of the total collection by number), and 98.04 ind./m² (73.80%), respectively. The numerical abundance of small jellyfish was highest in summer ((337.33±55.68) ind./m²), lowest in winter ((113.69±32.72) ind./m²), and intermediate in spring and autumn ((142.09±67.71) ind./m² and (132.84±35.17) ind./m²). Seasonal variations in the abundance of small jellyfish were closely associated with the surface water temperature. The average surface water temperature peaked in summer ((27.26±0.02) ℃). Then, both the temperature and salinity in the studied area were suitable for the development and reproduction of D. chamissonis. In addition, the summer upwelling and its associated high productivity might also benefit the jellyfish because large numbers of carnivores ultimately require an elevated level of primary production. There were strong correlations between temperature, 10-m water salinity, and small jellyfish abundances (P < 0.01), suggesting that they are important in determining the spatial distribution of small jellyfish. The bottom salinity in spring was positively related with the abundance (P < 0.05), and the 10-m water salinity in summer was also positively correlated with abundance (P < 0.01). In winter, the abundance was obviously increased from inshore to offshore. In addition, small jellyfish abundance was also strongly correlated with their food, small copepods (P < 0.01), which proved the importance of food availability for carnivores. This study provides basic knowledge on the distribution patterns of small jellyfish, which is essential to further understand the ecological roles played by small jellyfish and their responses to the hydrological conditions in the Changjiang Estuary and the adjacent East China Sea.