Abstract: Based on X-ray diffraction analysis of clay mineral compositions in the offshore surface sediments of Dalian Bay and Dayao Bay, together with separation and identification on light minerals, the assemblages and distribution characteristics of these minerals were studied. Their sources and sedimentary environments were discussed. Four kinds of clay minerals (illite, smectite, chlorite and kaolinite) are identified, whose average contents are 59%, 20%, 16% and 5%, respectively. Illite is abundant in the west part of the study area, while chlorite in the east. Smectite content is higher in the bays. Light minerals mainly consist of plagioclase (with an average of 45%) and quartz (34%), followed by K-feldspar (10%), chlorite (8%), muscovite (2%) and calcite (0.4%). They are distributed bandedly from east to west, which is consistent with the distribution pattern of grain sizes. The mineral distribution is obviously controlled by local topography. The coastal shallow-water slope in the north of the study area is characterized by enrichment of plagioclase, quartz and K-feldspar, and lower weight percentage of light minerals, indicating strong hydrodynamic conditions and nearby source. With water depth increasing southward to a submarine platform, there are high contents of chlorite, muscovite, rich clay and silt in sediments, and lower contents of plagioclase, quartz and K-feldspar, indicating weakened hydrodynamic conditions and far sediment source. Calcite is a characteristic mineral of Yellow River sediments and mainly distributed in the western study area, which illustrates that sediments in the western study area are partly from the Yellow River. According to the distribution patterns of chlorite, muscovite and clay, fine grained sediment zone in the middle of the study area is transported from east to west. To the further south of the study area with the deepest water off another underwater slope, contents of plagioclase, quartz and K-feldspar are high, while those of chlorite and muscovite are low, implying strong hydrodynamic conditions induced by tide current which passes in to and out of Bohai Sea and Yellow Sea Warm Current (YSWC) that enters Bohai Sea. In addition, larger weight percentage of light minerals shows that the sediments are transported a relatively long distance from source area compared with coastal sediments.