Although the Korean population of Eurasian otter (Lutra lutra) was designated as an endangered species, habitat and basic ecology of Korean otters have been rarely known. To ascertain sprainting behavior of the Eurasian otter in the freshwater habitat and investigate the habitat preferences, otter surveys along the Ungok River in Gyeongsangbuk-do, South Korea, were conducted for a 12-month period during 2002 and 2003. Seasonal fluctuations of the number of spraints and sprainting sites along the river were identified. The number of spraints and sprainting sites peaked in March. General linear models indicated site-dependency in the numbers of spraints and sprainting sites. The general linear mixed model indicated that the numbers of spraints (or sprainting site) were positively correlated with vegetations on riverbanks and river depth. Although human impact and river width were significantly related to the number of spraints, they did not show significant relationship to the number of sprainting sites. Despite several coincidence with studies in Europe, sprainting behavior by otters along a montane river in South Korea suggested differences in regional and spatial level. This ecological information on the Korean population of the Eurasian otter will assist the investigation of population size and help habitat management for otters.