In this paper, field measured values of soil temperature in four different soil conditions were used to calculate the apparent diffusivities by use of six methods, and the characteristics of every method and the quantity and quality of data required for calculating the thermal diffusivity of soil were analysed. These results show that four of the six methods (i. e. Amplitud, Phase, Arctangent and Logarithm) provide explicit equations for thermal diffusivities. These explicit equations only need a few measurements of soil temperature, and the calcutations are very simple, but the calculating results are erratic. Although numerical and harmonic methods must use implicit equations to calculate the apparent thermal diffusivities and need a great number of soil temperature data, their estimations are steady and reliable. The temperature gradients of soil simulated by the apparent thermal diffusivities under four different soil conditions were consistent with the field measurements, the standard diviation was less than 0.043. In this paper, that the harmonic method was also used to calculate the heat flux of four soils is with great satisfaction, as compared with the field measurements.