In the southern Yellow Sea in winter, a thermohaline front is formed where warm and saline water of Kuroshio origin meets cold and fresher coastal water of Yellow Sea origin. The detailed frontal structure as well as the northwest intrusion of the warm water across the front was investigated by analyzing conductivity-temperature-depth data and tracks of drifting floats observed twice in February and December 2001, historical moored current data, and satellite infrared images. During December – March, a tongue-shaped front is firmly established, with its tip heading northwest. A narrow warm water band often develops northwestward along 50 – 70 m isobaths on the western flank of the central trough from the northwestern frontal zone. In the February survey, floats in the warm tongue moved northwestward and westward after crossing the northwestern frontal zone where the frontal structure was nearly collapsed, reflecting the warm water intrusion across this frontal zone. The development of the tongue-shaped front at the Yellow Sea entrance may be closely associated with the response of the combined geometry of the Yellow and East China Seas to the strong northerly wind.