Abstract：37 diatom species, belonging to 21 genera, have been found from the surficial sediments in the north-central Bohai Bay. The distribution of nine dominant species, such as Cyclotella striata/stylorum, Paralia sulcata and Coscinodiscus perforatus, has been determined. Based on cluster analysis, the authors have divided the study area into six diatom assemblage-regions. Furthermore, environmental factors controlling the diatom distribution are discussed in this paper. It is revealed that salinity and water depth are two most important factors while the grain size of the sediments also has some influences on the distribution.