We studied copepod assemblages in the northern South China Sea during October 2004. A total of 78 copepod species was identified from 24 families and 42 genera that include the four orders Calanoida, Cyclopoida, Harpacticoida, and Poecilostomatoida. The high copepod diversity in the study area may mainly be caused by both the year-round Kuroshio Current intrusion and the SW monsoon, prevailing in the South China Sea during summer. Water currents in the region transport marine planktonic fauna, that comprises predominantly Copepoda from subtropical and tropical water masses. Calanus sinicus did not appear, indicating that there was no cold water mass intrusion in that area during October 2004. Clausocalanus furcatus and C. arcuicornis were most dominant, occurring in 27 of a total of 28 samples (occurrence rate (OR): 96.43%). Thirteen copepod species were found once in all samples. The three most dominant copepods considering all samples were Clausocalanus furcatus (RA: 15.32%), C. arcuicornis (RA 13.52%), and Paracalanus nanus (RA: 9.28%). Most species of copepods performed a common diel vertical migration, descending during daytime and ascending at night at different rates to different depth zones. There were smaller numbers of species and lower diversity indices in surface waters (1-2 m depth) as compared to depths below 50 m.