We analyzed the characteristics of fog formation in the Gumi Reservoir of Nakdong river with the field observation data for recent 2 years (1 April 2013~31 March 2015) collected by the national institute of meteorological research, KMA. In early morning, we frequently observe the steam rising from the water surface. The fog occurs from adding water vapor into the air. We call the fog as steam fog. Steam fogs occur when cold, dry air mixes with warm, moist air above a water surface. The steam fog appears mainly in autumn under the following conditions; (1) sensible heat is positive values (10~20 W/m2), (2) latent heat is more positive values (25~35 W/m2) than sensible heat, (3) cloudless nights with light winds (about 1.5 m/s), (4) under condition(3), mountainous winds easily blows into the reservoir.