Abstract: In base of the data of 4 comprehensive investigations on the ecological environment in Beibu Gulf during 1998.1~1999.5, we analyzed the seasonal species composition and diversity of planktonic Copepods during El Niño/La Niña, exploring the relation between them and marine hydrology. The results reveal that 122 species of Copepod are identified, among which 32 species occurred yearly. The species of Copepod are found relatively more in spring (82) and summer (84), while less in autumn (52) and winter (67). Among the 13 dominant species in 4 seasons which are mostly eurythermal and euryhaline, 12 species are seasonal species except Canthocalanus pauper. The meso-copepods, the dominant species in winter and summer, are affected by El Niño. The macro-copepods, which take relatively high proportion in autumn and spring, are influenced by La Niña. The Copepods in the area was dominated by wide-temperature and wide-salt species. The diversity of the planktonic Copepods in Beibu Gulf is relatively high. Water temperature and salinity may affect the community diversity of Copepods in summer, but the impact is not obvious in the other 3 seasons.