On the basis of the hydrographic data in the area west of Kyūshū from four cruises of R/V Xiangyanghong No. 9 from 1987 to 1988, the circulation features in the investigation area are described and the source of the Tsushima Warm Current water (TWCW) is explored by using the observed and geostrophic current results and tracking the Kuroshio Subsurface High Salinity Core (KSHSC).