This paper examined the impacts of dam construction on fog characteristics over surrounding areas. Juamdam which only supplies the water for use and Chungjudam which generates electricity were selected. The number of foggy days, fog occurrence and dissipation time and the differences of each lake water temperature and air temperature at Sunchun and Chungju were analyzed. After the construction of dam, the relative humidity and water vapor pressure were decreased at Sunchun and Chungju. The number of foggy days did not increase at Sunchun while largely increased at Chungju. Just because Juamdam were contained water, the water vapor pressure at surrounding areas of the weather station were largely decreased. It delayed the time of fog occurrence by bringing out the decrease of steam fog. The foggy days increased over the Chungju area due to the difference between air and cold outlet water temperature. The increase of foggy days mainly resulted from evaporation during colder seasons and from the temperature inversion over the water surface during warmer seasons.