The detection of fog is very important for application of traffic safety over land and sea. Geostationary satellite can provide the excellent information for this need due to its high temporal resolution. Especially, however, it is difficult to detect fog during the nighttime because the thermal contrast between the fog and the surface as well as the fog and low stratus clouds is usually very small in infrared images. The objective of this study is to develop an algorithm to discriminate the fog from either ocean or low stratus clouds using geostationary satellite-MTSAT. From the analysis of the variation of 11 µm surface temperature as well as 11-12 µm brightness temperature difference over the ocean, we have found that the threshold values for background temperature and the temperature difference can be determined. Based on tracking the deviation of the threshold values over the course of nighttime, we have examined the possibility of detecting the location and evolution of seafog over Korea.