The method of gradient transformation was initialized in the paper. The diffusion coefficient in the governing differential equation of mass transfer and the surface emission coefficient on the boundary condition were experimentally determined. The effect of undersurface evaporation was quantitatively analyzed. The relationship between relative evaporation rate and the relative depth of evaporation surface was given. The experiment showed that the relative depth of evaporation below 1% influenced the relative evaporation rate significantly. The surface emission coefficient was also a function of the relative depth of evaporation.* The diffusion coefficient was a function of moisture content. By taking into account of the dependent of the above mentioned coefficients on the relative depth of evaporation and moisture content, the reliability of the given coefficients was tested by using the analytical solution of the governing differential equation of mass trapsfer. This showed a deviation in less than 6 between the measured values and the calculated values, which posed the results of the study as a valid exploring.