Natural ventilation system modifications were evaluated using a computational fluid dynamics (CFD) numerical model (code: Fluent Version 4.5). Wind speed and direction, side vent opening size and location, roof vent opening type, and number of spans were examined in terms of ventilation rates and airflow distribution. For the side vent located at 2.5 m above the floor with a west wind of 2.5 m/s, 59% of the incoming air through the side vent was predicted to move out through the first roof vent opening without reaching distant areas in the greenhouse, resulting in high inside air temperatures. The air mainly moved in through the side vent and fourth roof vent openings for an cast wind of 0.5 m/s while the third and fourth roof openings were the only predicted inlets of airflow for an east wind of 2.5 m/s. The hinged open roof multi-span greenhouses were predicted to have significantly higher natural ventilation rates than the double polyethylene-covered multi-span greenhouses for all the spans in the absence of side vent.