Observation and division of sedimentary facies, and establishment of molluscan fossil assemblage in 5 sediment horizons of the early Middle Miocene sediment body (Shinhyeon Formation of Ulsan Basin) distributed at Kangdong-dong, Ulsan City, conducted to understand the mechanism of development and changing history of molluscan fossil assemblages. Sedimentary facies and Molluscan fossil assemblages composed of composed of 7 types (facies 1-7) and, Crassostrea assembage (layer I) - Vicarya-Cultellus assemblage (layer II) - Crassostrea assemblage (layer III) - Vicarya-Cultellus asemblage (layer IV) - Vicarya-Tatewaia-Striarca assemblage (layer V) toward upper from lower layer, respectively. Crassostrea assemblage of layer I and III indicating the intertidal-subtidal environment and Vicarya assemblage of layer II, IV and V indicating the marsh-intertidal environment appeared repeatedly. It is caused by the transgression and regression of sea level.