///The clay minerals of 166 surface sediments and 23 sediments of sediment core A435 from the northern part of the Bohai bay were analyzed with the X-ray diffraction method. The results showed that Illite, averaging 53.0%, was the dominant clay mineral in the surface sediments of the northern part of the Bohai bay, followed in concentration by smectite (27.9%), chlorite (9.9%) and kaolinite (9.2%).The illite-smectite-chlorite-kaolinite type was the main clay mineral assemblage of the surface sediments. In the sediment core A435, the assemblage of clay mineral varied greatly above the layer of 100 cm. That’s may be caused by the migration of the Yellow River, which discharged into the Bohai from 1048 to 1128 AD and emptied into the South Yellow Sea from 1128 to 1855 AD.oft-com:office:office" />