Energy balance methods currently used to estimate the natural ventilation rate of greenhouses ( ma  ) assume that the input energy to the greenhouse is the transmitted solar radiation. An average value for the cover transmittance ( c  ) is usually used to estimate this energy. However, this estimate includes a large error due to the spatial variation of c  in the greenhouse. These methods also give negative values of ma  at low solar radiation levels (e.g. in the morning and afternoon). The purpose of this study was to develop a simplified energy balance model to estimate the value of ma  precisely. In this model, all modes of energy were treated at the outer surface of the cover to avoid the error caused by using an average value of c  . Required environmental parameters to be used in the model were measured inside and outside a single-span glass-covered greenhouse with a floor area of 26 m2 located in the Tokyo area, Japan during four sunny days (Sept. 29 to Oct. 2., 2005) The greenhouse was naturally ventilated using two roof ventilators (0.6 m  5 m). Diurnal variations of ma  were estimated and compared with the results of other models and showed the necessity of applying the present model. The estimated value of ma  was in the range between 0.11 kg s -1 and 2.25 kg s-1 and these results were in accordance with those measured and reported in the literature. The results also confirmed that outside wind at a speed less than 2.5 m s-1 has no significant effect on the value of ma  and the value of ma  resulting from the greenhouse thermal balance depends mainly on temperature difference of air between inside and outside the greenhouse ( T ). A linear correlation between ma  and T was provided that can be used to estimate the required ventilation rate to maintain the air temperature in the greenhouse at a desired level.