Abstract: The annual Diel Vertical Migration (DVM) patterns of Zooplankton in Jiaozhou Bay were studied in four seasons during 2006 and 2007. The station was situated at 36°5'54″N,120°15'20″E, and samples were collected diel-continuously during 24 h in one day. The results showed that zooplankton acted differently during four seasons. In spring, they swam downward at noon and midnight, i.e. they acted as double cycles of rising and sinking, while in summer, zooplankton showed with the spring oppasite. In autumn, the center of density had a broader range than any other season, zooplankton seemed to stay at the upper layer at daylight, and to sink at night. In winter, zooplankton showed weaker DVM behavior, and the law was with the spring opposite.We studied the DVM characteristics of dominant species in Jiaozhou Bay and classified them into different groups as well. It was found that the ways and the time of the DVM for different species were different in the same season. As for the same species, they usually changed with seasons. As a result, for zooplankton, the DVM was always changing, it was determined not only by exogenous stressors but also by endogenous rhythms. And the key factors which contributed were not the same in four seasons. Sunlight was one of the key factors for all seasons. Besides, In winter, temperature, salinity and phytoplankton were also important. In total, zooplankton showed positive relationships with temperature and phytoplankton, and a negative relationship with salinity. Moreover,for specific species, the key factors which were responsible for the pattern of DVM were different.