The hydrographic data collected at three different times July, 1994, May, 1995 and June, 1996 around Taean peninsula in the mid-Yellow Sea off Korea, well known for the well-defined surface thermal fronts in summer, were analyzed. In the vertically well-mixed area where water depths varied from 15 m depth to 60 m depth, the temperature difference in the water column was less than $1^{circ}C$. The temperature observed in the vertically well-mixed area was reversely related with the water depths and the coldest surface water was always observed over the deep channel with the depth of more than 50m, which developed southwestward off the promontory of Taean peninsula, irrespective of the observation period. The strengths of surface thermal front observed in June were much stronger than those in July, even though the surface temperature of stratified area were nearly the same as in July. These observed features could be explained as follows: A major physical process for the formation of the surface thermal front is the vertical mixing of water column but the detailed thermal structure in the study area depend on the physical parameters such as the water depth in the vertically well-mixed side and the vertical thermal structure in the stratified side.