Abstract: he diatoms of borehole CH19, located at the shallow sea area, northwestern Bohai Bay, have been systematically studied and, totally, 36 species in 17 genera were identified. Consequently, Ⅰ—Ⅶ diatom zones have been divided according to the characteristics of the diatom assemblages, the sediments and the OSL and AMS 14C chronological framework of the borehole as well. The diatom and chronostratigraphical results reveal that the study area was changed from the lacustrine to marine environment in the Holocene. The diatom Zone Ⅰ, indicating the beginning of the Holocene, 11ka cal BP, was the supratidal swamp environment. From 10ka cal BP, the area entered a long period of marine influencing environment, during which the zone Ⅱ, occurring around 10 to 7ka cal BP, was the intertidal zone; Zone Ⅲ indicated that a 1.7m-layer, consisting of mixture of unarticulated shells, shelly fragments, sandy and muddy sediments, was affected by storms during 6.77~3.47ka cal BP in the Middle Holocene; Zone Ⅳ returned back to the normal shallow sea environment; Zone Ⅴ was unstable again; Zone Ⅵ was characterized by decreasing salinity; finally, Zone Ⅶ indicates the modern diatom assemblage on the bottom of the shallow sea, northwestern Bohai Bay.