Abstract: A total of 3 phyla 60 genera and 114 taxa(not including undefined species) were identified by the Utermöhl method in the survey area. The major groups were diatom and dinoflagellate. The ecotypes of most phytoplankton were temperate and coastal species. The dominant species were Dictyocha fibula, Gymnodinium sp., Gyrodinium spirale, Diploneis bombus and Paralia sulcata. The cell abundance of phytoplankton ranged from 0.037 0× 10³ to 32.3 × 10³ cells·L^-1, with an average of 2.04 × 10³ cells·L^-1. The cell abundance of phytoplankton decreased offshore. Cell abundance was highest in west region of the northern Yellow Sea, about 32.3×10³cells·L^-1, and was also relatively high in southeast region in the survey area. The vertical distribution of phytoplankton was generally decreased with the increasing of water depth, and gradually decreased from coast to offshore. The Shannon-Wiener diversity was high in the southwest survey area, and the Pielou's evenness index was high in the eastern survey area. According to canonical correspondence analysis (CCA), the main factors that influence the abundance distribution of dominant species are nutrients, and the correlation with salinity and temperature is not obvious.

Key words: phytoplankton, community structure, South Yellow Sea, summer