In a field study, we investigated the effect of seed-bed preparation methods and mulch materials on soil temperature. Seed-bed preparation methods involved ridges and flat surfaces, and mulch materials were rice straw and clear plastic sheets used on flat surfaces. In ridges 1 cm deep, the maximum soil temperature was 6°C higher and the minimum was 2.5°C lower than that observed in a flat plot. Plastic mulch on a flat surface lowered the maximum temperature at the 1-cm depth by 5°C, and rice straw mulch lowered it by 16°C, compared with bare flat. These differences decreased with depth. Regression equations between soil depth and the differential of the maximum or the minimum soil temperature at any depth and corresponding air temperature 1 m above ground level were developed. The correlation coefficients varied between 0.98 and 0.99. The soil temperature computed from the regression equations utilizing air temperature was in good agreement with the observed. Fourier series analyses of soil temperature showed that the first two harmonics could describe the temperature at the 5-cm depth satisfactorily and that the first harmonic alone could do so at the 35-cm depth.