The characteristic of cold water by upwelling in the southeast of the Korean Peninsula and the effect of sea breeze in this region are investigated. The coastal upwelling around the southeast coast of the Korea Peninsula is analyzed by using Coastal Oceanographic Data statistical analyses for 5 years were carried out. The period of an cold water event, on the average, was observed southwesterly wind events. The analysis suggests that strong and persistent southwesterly winds in period of an cold water play an important role of bring the moisture to the surface, generating persistent cyclone as jangma. In order to investigate the effect of cold water on sea breeze, we considered two case. First, Exp. 1 is not occurred coastal upwelling on sea breeze. Second, Exp. 2 is occurred cold water on sea breeze. Two experiments were completed separately to the effects of cold water by upwelling. The results show the sea breeze is stronger in Exp. 2, when the cold water occurs, and weaker in Exp. 1, when there is no cold water. In order to verify the effect of the sea breeze on the cold water by upwelling, on the intensification and change of direction of the prevailing wind, the sea breeze effectively intensify cold water condition.