In a greenhouse cooled by a fogging system, the fraction of fog that evaporates by absorbing sensible heat from the greenhouse air, beta, is an essential parameter to be determined for evaluating the system performance. Recent studies estimated beta under indoor conditions by collecting the non-evaporated fog that fell on a plastic sheet during a specified time and beta was the difference between the amount of sprayed fog and the non-evaporated fog divided by the amount of sprayed fog. Using this method in the greenhouses causes an overestimation of beta due to the evaporation of the fog that fell on the plant leaf/floor surfaces affected by solar and thermal radiation and the difficulty of collecting the non-evaporated fog that falls on the plant leaf surfaces. This paper presents a method for simulating beta and for analyzing the fog evaporation based on the heat and water vapor balance of the greenhouse air. The conditions of the un-cooled air in the greenhouse were investigated to be used essentially in the simulation. An experiment to determine parameters for the simulation was conducted on a hot sunny day (August 9, 2004) in the Tokyo area to measure the environments inside and outside a naturally ventilated greenhouse with a floor area of 26 m(2). The greenhouse was cooled intermittently at a fogging rate of 10 g s(-1) for five different fogging durations (i.e., fogging time-interval time) of 1-3, 0.5-1.5, 1.5-4.5, 0.5-1 and 1-2 min, respectively. Evapo-transpiration rate of 150 potted tomato plants was estimated using reported correlations and the resulting values could be corrected based on the water vapor balance of the greenhouse air. The results showed that beta had a certain pattern along with the fogging and interval time. The values of beta estimated from the heat balance were identical to those estimated from the water vapor balance. The fogging duration of 1-3 min showed relatively high evaporation rate, in which, the integrated value of beta over a working time of 41 min was 0.36. (c) 2005 Elsevier Ltd. All rights reserved.