The author discussed the late Holocene peatland development in the Kutonebetsu River Lowland, a riverine-coastal lowland, in relation to relative sea-level changes. The Kutonebetsu River Lowland locates in the western coast of the northern part of Hokkaido Island. Discussions are based on results from the interpretation of aerial-photographs, hand-boring surveys, radiocarbon dating using accelerator mass spectrometer (AMS) and FeS2, content analysis of the sediment. ### Two Holocene geomorphic surfaces can be recognized in this lowland. The Holocene sediments consist of a thick clay layer and a 2- to 3-meter-thick herbaceous peat layer overlying basal Neogene mudstone. Judging from the results of FeS2 content analysis, the upper part of the clay layer was deposited in brackish-water environment and the peat layer was deposited in fresh-water environment. ### Late Holocene geomorphic changes of this lowland can be summarized as follows ; ### In the middle Holocene, relative sea-level reached to a little-higher than the present sea-level, and brackish lagoon spread behind coastal barrier. After about 4,000 yrBP, peatland (upper Halocene surface) began to form from the edge of the lagoon. In about 2,900 yrBP, when relative sea-level probably fell, the water-level of the lagoon also dropped and peatland rapidly expanded to lagoon. After that, the river dissected the older surfaces and developed floodplain (lower Holocene surface). ### This geomorphic changes reveal that late Holocene sea-level fluctuation was important factor for the peatland formation in this lowland, and that the tectonic uplift probably occurred in the western coast of northern Hokkaido during the late Holocene.