Heat flux in the surface sediment layer at the west coast Korea was investigated using one year-long temperature data collected on two tidal flats of the Guenso Bay and Seonyu-do. The temperature data was measured at six depths from 0 cm to 50 cm, at a 10 cm interval. The sediment temperature shows large annual variations, high in summer and cold in winter and also high-frequency fluctuations at tidal frequencies of 1, 2, 3, and 4 cycles per day. In summer, temperature decreases with increasing depth, but in winter, vertical temperature profile is reversed. Vertical thermal diffusivity computed by an one-dimensional heat conduction equation is found to be largest in spring. We will further discuss on the dependence of the thermal diffusivity on grain size of sediments and diurnal heating-cooling condition.