On basis of the analysis of the temperature and salinity data observed in July-August, 1972 and some historic data of the temperature and the salinity, the authors found that there was frequently an isolated cold water mass at the middle and upper water layers in the northern part of the East China Sea (SW of Cheju Island) in summer. After comparing the phenomenon with the residual current data observed in this region, the authors pointed out that there was a cyclonic eddy in this region. It is shown from calculation and analysis that the horizontal extent of the upper portion of the eddy is small, but that of the lower portion large.Its average horizontal scale is about 90 miles,the average velocity of upwelling at its center approximately equals to 4.3×10-3cm/sec. The authors suggested that this cyclonic eddy seems to be a small secondary circulation of the anticlockwise circulation in the Yellow Sea and the East China Sea.Its formation is closely related to the topography of the bottom of this region, the appearance of cold water from lower layers in summer and the shear effect of two current systems of the Yellow Sea Warm Current and Coastal Current of the Yellow Sea.