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A computer simulation model for natural ventilation in open-roof greenhouses was developed to predict the ventilation performance. The model predicts ventilation rate and the temperature differences between inside and outside, based on the weather and structural conditions including internal net radiation, wind velocity, and height and area of the roof openings. The ventilation rate was calculated from thermal buoyancy and wind forces. A sensible heat balance was incorporated to calculate the ventilation rate and the temperature difference simultaneously. A four-span open-roof greenhouse with roof sections hinging at the gutters and opening at the ridge, constructed on the Rutgers University campus, was used for data collection. Measurements of climate conditions in the direct vicinity of the greenhouse were conducted. Using the observed outdoor and greenhouse conditions, the model parameters were calibrated statistically. The accuracy of the model and the modifications to the model are discussed by comparing the predicted and observed greenhouse temperatures. It is shown that the internal temperature rise depends on the roof configuration as well as solar radiation and wind velocity. The resulting simulation model can be used to implement new environment control strategies for open-roof greenhouses.