The present paper evaluates the thermal load reduction of air-handling equipment achieved by energy-efficient hybrid ventilation systemsfor office buildings through simulations, using the program Building Energy Simulation Tool (BEST), a transient solver developed in Japan.The target ventilation systems are automatic control systems that integrate natural ventilation and mechanical ventilation, including air-sideeconomizer control, demand control, and heat recovery control. In advance of the simulation analysis, a thermal load simulation method wasproposed for the evaluation of energy-efficient hybrid ventilation systems. Simulations throughout the year for 836 cities in Japan and eightcities in the US were then performed, and the regional features of ventilation operation and the equipment load reduction achieved by energyefficienthybrid ventilation systems were evaluated.