Abstract: Eighteen microfossil samples were taken from borehole YZ1 at Yangzi Town on the southern coast of the Laizhou Bay of the Bohai Sea. Based on the stratigraphic correlation and analysis and identification of the genera,abundances and di- versities of microfossils,four fossil assemblage zones are distinguished. According to the characteristics of microfossil assemblages, the authors think that microfossil assemblages II and I are products of Holocene marine transgression. Assemblage I represents a transgressive sequence and the sedimentary environment is marked by the supratidal zone- intertidal zone. Assemblage II repre- sents a regressive sequence and the sedimentary environment is marked by the supratidal zone- subtidal zone. Assemblages III and IV are products of two late Pleistocene marine transgressions respectively. Assemblage III is the product of the latest Pleis- tocene transgression and indicates a large scale of transgression and a tidal flat- intertidal environment, whereas assemblage IV shows a relatively small scale of transgression and an increase in continental factors and indicates a river mouth- supratidal or paralic environment.