This paper describes the mechanism of evaporation in a sand column under a diurnal temperature variation. Thermo-hygro meters were used to obtain the vertical profile of water vapor density along a sand column. The correlation between air temperature and evaporation rate is positive and the evaporation rate is high when the air temperature is high. There is, however, a time delay between these two time series and it mainly depends on the time change of the water vapor density at the top of capillary zone. As the dry layer becomes thick or the level of groundwater table becomes low, the time delay gets larger. It is concluded that the diurnal evaporation variation can be well explained by the diffusion theory of water vapor.