Data presented in this paper were obtained during four cruises conducted in the East China Sea (23°30′～33°N, 118°30′～128°E) during four different seasons from 1997 to 2000. Ecological characters of the population of a copepod, Eucalanus subcrassus in the East China Sea was analyzed using different methods, such as ecological index and the variance contribution analysis, and further compared with the historical data of 1979～1980. Results show a clear seasonal pattern of the E. subcrassus with an abundance on average: autumn (5.40 ind/m³) > summer (2.09 ind/m³) > winter (0.93 ind/m³) > spring (0.17 ind/m³). The dominance of E. subcrassus also changed seasonally with the same tendency as its abundance, autumn (0.10) > summer (0.08) > winter (0.07) > spring (0.01). As a dominant species of copepods in the East China Sea, E. subcrassus predominated in summer and autumn. The percentages of the E. subcrassus in total abundance of copepods were 9.94% in the summer and 9.32% in the autumn, respectively, compare to the lowest (2.46%) in the spring. Occurrence was the highest (98.22%) in the autumn, then the winter (84.62%) and the lowest (49.62%) in the spring. E. subcrassus was the second dominant species in copepods in the summer, fall and winter. Its contributions to total abundance of copepods (β) in summer and winter were 0.39 and 0.27, respectively, following Calanus sinicus (0.88 and 0.45), and 0.46 in autumn that was only lower than Euchaeta concinna (0.56). In thespring, the occurrence was very low. Another character, indices of clumping (I) of E. subcrassus in four seasons were positive. The index was higher in autumn and summer. In spring, E. subcrassus mainly distributed at the Taiwan Strait where water temperature was higher than other zones. As water temperature rose, E. subcrassus moved toward north and distributed widely in waters where water temperature and salinity were higher than 24.0℃ and 32.5‰, respectively, from summer to autumn. The variation of its abundance was associated to bottom water temperature, which was only temperature-dependent in spring and summer, surface and bottom salinity as well as bottom temperature related in autumn, and neither temperature nor salinity associated in winter. Comparison between the data presented in this paper and the data obtained from 1979～1980 indicates that the average abundance of E． subcrassus in 1997 was 10 times higher than in 1979; the dominance and percent in total abundance of copepods in 1997 was higher than in the autumn, winter and summer of 1979; the occurrence in the winter and autumn of 1997 also was higher than in 1979, but in the spring. This phenomenon might be related to global warming, because the bottom water temperature and salinity, the key environmental factors that affect the abundance of E. subcrassus was closely related to warm current. The close relationship between the abundance variation of E． subcrassus and warm current in the sea suggests that E. subcrassus is a warm water species in the East China Sea.