The 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 Holocene.