Diatoms are a primary producer and play an important role in the functioning of microbial food webs. Temporal variations in community patterns of planktonic diatom assemblages were studied during a 1-year cycle (June 2007–May 2008) in Jiaozhou Bay, northern China. Samples were collected biweekly at a depth of 1 m from five sampling stations. A total of 75 diatom species representing 40 genera, 28 family, 19 orders and three classes were recorded. Of these species, 11 distributed in all four seasons, while 27, 35, 56 and 28 forms occurred only in spring, summer, autumn and winter season, respectively. The species number and total abundance peaked in autumn, with minimum values in May. All three species biodiversity measures (Shannon diversity, Pielou's evenness and Marglef's richness) peaked in spring and autumn. There was a significant difference in diatom community patterns among seasons, except the pair of spring and winter. The environmental variables, especially temperature and the nutrients, could significantly drive the seasonal variation in diatom community patterns. Of 11 dominant species, four (Paralia sulcata, Skeletonema costatum, Guinardia delicatula and Nitzschia lorenziana) were significantly related with temperature, pH and/or nutrients. These findings suggest that the seasonal shift in community pattern of planktonic diatoms was driven by both temperature and nutrients in this eutrophic basin ecosystem.