Abstract: Based on systematic open Ilume flow experiments, this paper investigates the basic lows of thermal diffusion coefficients in water flow. The analysis of testing results show that the proper scale of longitudinal dispersion coefficients, related to the water surface slope and Froude number, is the nonhomogeneous compre-hensive factor of velocity distribution. A formula for calculating tte thermal longitudinai dispersion coefficient in water flow is proposed. Meanwhile, the authors found that the coefficient of thermal transverse diffusion has the similarities to that of chemical matter-diffusion. Finally, the formulae for estimating the uncertainties of temperature measurements and longitudinal dispersion coefficients are derived by using error theory.