Adequate control of greenhouse environmental parameters is very important for successful plant growth and development. Typical methods for preventing excessive temperature increases are ventilation to exchange internal air with cooler external air, shading to decrease incoming solar radiation, and evaporative cooling. Particularly, ventilation is one of the most important techniques for controlling greenhouse environments, and natural and mechanical (fan) ventilation systems have been widely adopted. Natural ventilation relies on thermal buoyancy and external pressure differences resulting from local wind conditions. However, even in mild climates during the summer, high ventilation rates are required to prevent excessive internal temperatures, but it is often too expensive to realize such ventilation rates with mechanical ventilation. Larger ventilation openings can be installed to improve the natural ventilation performance. One of the recent innovations in greenhouse structures is the open-roof design, which was introduced during the 1990s. Adequate greenhouse temperature control is also important in order to reduce thermal stress on greenhouse workers. The effects of vent design on natural ventilation and the resulting temperature difference between the inside and outside of one typical arched-roof greenhouse and two open-roof greenhouse designs were studied experimentally during the summer. The roof vent design investigated were: An arched-roof vent type (AVT), roof sections hinging at the gutters and opening at the ridge (RVT), and roof sections sliding on a lattice beam and opening at the gutter (GVT). When the roof vents were fully opened in the RVT and GVT designs, the temperature differences between the inside and outside air were significantly lower than when the roof vent was fully opened in the AVT design. Planned future work will result in an environmental control system for inside solar radiation, temperature, relative humidity, and carbon dioxide concentration inside open-roof greenhouses.