A mathematical model to simulate evapotranspiration was developed and used to calculate the evapotranspiration from a small soybean field. The calculated evapotranspiration rates were compared with the estimation obtained by the energy balance method. Observations were carried out on sunny days from late July to August in 1976. The analysis of the data at the 12th August is presented in this paper. When the energy balance method is applied to a small field, advective energy from surroundings must be considered. In such cases, the equation to estimate the evapotranspiration rates should be modified as ET_0 =[(S-G)+ρ∫^H_H_0 u(Bλ(∂q/∂x) – c_p (∂T/∂x) dz ] /(1+B) (1) where S is the net radiation, G is the soil heat flux, p is the density of air, u is the horizontal wind speed, B is the Bowen ratio at some height H, A is the latent heat of vaporization of water, q is the specific humidity, c,, is the specific heat of air, T is the temperature, Ho is the crop height and x is the fetch. Plentiful water was irrigated before measurements to get rid of the stress on water uptake by root system due to soil water deficit. Hourly evapotranspiration rates were calculated with the equation (1) and compared with the results obtained from the conventional energy balance method. The contribution of advection to the evaportanspiration was estimated to be 5 % at its maximum. The value was rather smaller than that of expected because surroundings were vegetated fields such as sweet potatoes, corns, clovers and so on. Applicability of the proposed model for estimating evapotranspiration was examined. In this canopy model, following equations were included: S(ζ) = S_exp [-α∫^ζ_0 F(ζ)dζ] (2) K(ζ) = K_exp [-β(ζ/H_0)] (3) and r_s = r_min