Abstract: Among the oyster reefs on the northwest plain of the Bohai Bay,the Lingtou and Dawuzhuang are the two being carefully studied. 32 species in 18 genera diatoms were identified from the Lingtou oyster reef and the overying muddy sediments. From bottom to top, three diatom zones have been identified indicating that the marine influence had faded out during the oyster reef growing. The re-study of the diatom assemblage of the Dawuzhuang oyster reef, 20 km away form Lingtou, and the comparison of the diatom assemblages between the two reefs indicates that the Dawuzhuang oyster reef contains more diatom species with a big percentage of the intertidal-estuarine and coastal species.It suggests that the reef was then living in an intertidal zone or a shallow sea next to the estuary. It was under the influence of both the river and the sea, and the percentage of C.stylorum and C.striata indicates the change in river influence. On the contrary,the Lingtou oyster reef has fewer diatom species and a big percentage of marine and plankton species.It means that the reef maybe lived a little far away from the estuary or in a closed lagoon or a semi-clased bay.It was influenced by sea only, and the percentage changes in P.sulcata and T.nitzschioides indicate the fluctuation of marine influences.