Abstract: Diatom study on core 17940 from the northern slope of the South China Sea (SCS) reveals that subtropical planktonic species are the most important components in the diatom assemblages during the latest Pleistocene. The changes in the diatom assemblages indicate that the SCS at that time was more closed and the sea level was lower than at present. Meanwhile, the Bølling-Allerød wet phase and the cold and dry Younger Dryas event were recorded based on changes in some characteristic species. However, the boundary between Younger Dryas and Holocene in diatom record is not clear.