A theoretical model for IR radiance statistics of natural terrain was presented by using Monte Carlo simulation. The model is based on the heat balance equation of the natural terrain and the assumption that the short wavelength absorptance, long wavelength emissivity, wind speed and heat conductivity (for bare soil) or bulk stomatal resistance (for vegetation canopy) are random, uncorrelated variables with normal distribution densities.The results show that due to nonlinear coupling of the temperature to the thermal properties and heat source, the radiance distribution varies in a complex manner while the heat source changes. In the random variables selected, the influence of the field fluctuation in the short wavelength absorptance and wind speed on the statistical nature of the vegetation canopy is most obvious.