A cyclonic mesoscale eddy located about 150 km southwest of Cheju Islanl, the scales of which are 100-200 km horizontally and 50 m vertically, is reexamined with current measurements. The year round and multiyear existence of the eddy has been identified with thermal structure and current measurements. Interannual flucturation of the eddy in summer is discussed in somewhat detail in terms of closed isotherms at various levels. The observed current data in 1972’s summer agree well with computed geostrophic flow, i.e. they both show the existence of the cyclonic eddy with cold and saline water in the central area. It is reasoned that the formation of the eddy is closely correlated to the interaction of the Yellow Sea Warm Current, Yellow Sea Coastal Current and the northward residual of Kuroshio, and the Changjiang Dilutel Water may also play an important role in summer.. It is important to note that position and dimension of a distinct circular-shaped region of c1ayey mud is surprisingly coincident with those of this eddy.