Abstract: Based on two days survey in August,2001 in Jiaozhou Bay,the structural characteristics of phytoplankton community during harmful algae bloom was studied. The results showed that phytoplankton community was composed mainly by the coastal warm species, which were dominated by the diatom including37 species. Only 7 Pyrrorophya and 1 Chlorophyta species were identified. The highest value of species number appeared in the central of Jiaozhou Bay, which included 37 species, but the lowest value in the edge of Jiaozhou Bay was only 10 species. The highest value of cell abundance appeared in the eastern waters of Jiaozhou Bay, being 6.96?10⁸cell穖^-3,and the lowest value appeared in the mouth of Jiaozhou Bay,being 3.18?10⁶ cell穖^-3.The diversity and eveness of phytoplankton was the lowest in the eastern waters of Jiaozhou bay, but was higher in the mouth and the central of Jiaozhou Bay. Water mass movement and eutrophication were the main factors affecting the distribution of phytoplankton community in Jiaozhou Bay.