The distribution patterns of the benthic macrofauna of Han Estuary and Kyonggi Bay and the controlling environmental factors were studied at twenty-five stations in spring and fall of 1989. As a result, four biological groups were established as follows : Crassostrea gigas-Balanus reticulatus (Group I), Heteromastus filiformis-Mediomastus californiensis-Lumbrineris spp. -Sternaspis scutata-Tharyx sp. 1-Diopatra bilobata (Group II-A), Haustoriids-Phoxocephalids-Moerella rutila (Group II-B) and Nephtys chemulpoensis (Group II-C in March) and indistinctive group which was composed of common species (II-C in September). Results of the habitat analysis revealed that most of the dominant species showed narrow ranges of habitat niche in March and relatively wide ranges in September. Based on multiple discriminant analysis, the critical environmental factors governing their distributions are salinity in the regions of Yomha and Sokmo Channel in Han Estuary and sediment types in Kyonggi Bay. Also, sediment instability during the rainy season due to run-off was assumed to play a major role in the species composition of the benthic communities and their distribution in the study area.