The effect of a tidal flat on the dispersion of thermal effluent from the nuclear power plant at YongGwang (5.8 ¿ 106 kW) along the west coast of Korea was investigated using intensive field observations and three-dimensional numerical simulations. The calculated sea surface temperature distribution reproduces well the observed one with a root mean square error of 0.53¿C. A water column on the tidal flat area received a heat influx of 168 W m-2 on 17 April 2003 and transported the received heat to the adjacent coastal water at low water. The total heat supply from the tidal flat area to the coastal water was 6.6 ¿ 1010 W, and was larger by about 1 order of magnitude than that from the power plant of 7.0 ¿ 109 W on 17 April 2003, which was a fine day at spring tide. This fact shows that the effect of the tidal flat on the thermal effluent from the power plant is very large.