This paper presents the description and the performance curves of a natural draft solar fan that suits the ventilation requirements of greenhouses in hot climates. The developed solar fan utilizes the abundant sun energy (fan heating load) to induce upward air flow due to buoyancy forces and chimney thermal effect. The ventilated air is extracted into the greenhouse from outside through a wet pad, thus producing air cooling by means of evaporation. A back-up passive system is used for overnight and cloudy days. Different greenhouses - fan configurations and fan performance theoretical analysis are presented. The greenhouse ventilation air flow rate was found to depend upon the fan heating load, chimney height, greenhouse set temperature and ambient temperature. Computation results for a typical 3 m greenhouse height, 2000 m greenhouse ground area and 5 m chimney height indicated that the ventilation rate is 29 air volume change per hour AVCH (m = 11 kg/s), when the greenhouse is maintained at 25°C and the ambient temperature is 30°C. Reducing the ambient temperature to 20°C and 10°C increases the ventilation rates to 38 AVCH (m = 14.5 kg/s) and 44° AVCH (m = 17 kg/s) respectively. The present ventilation system is passive and therefore requires minimum maintenance, minimum operation skills, no electricity, and yet suits agricultural purposes in desert and remote areas.