To properly respond to demands to reduce national energy consumption and meet greenhouse gas emission targets based on environment policy, the Ministry of Trade, Industry, and Energy of Korea formed a research consortium consisting of government agencies and academic and research institutions to establish the first fuel efficiency standards for medium- and heavy-duty (MHD) commercial vehicles. The standards are expected to be introduced in 2017 as Phase 1 of the plan and will regulate trucks with a gross vehicle weight in excess of 3.5 tons and buses with a carrying capacity of more than 16 persons.Most MHD commercial vehicles are custom-made and manufactured in diversified small-quantity batch production systems for commercial or public use, resulting in difficulties in utilizing mandatory vehicle tests for fuel efficiency evaluations. Therefore, like Japan, the United States, and the European Union, where the simulation method has been adopted for MHD commercial vehicle fuel efficiency and greenhouse gas emission regulations, Korea has developed a vehicle system model, the Korean Energy Efficiency Simulator (KEES), as an official MHD commercial vehicle fuel efficiency certification tool.In this paper, the development process of the current version of KEES is mainly described. KEES calculates a vehicle's real-time fuel consumption in relevant driving scenarios based on the combination of both backward- and forward-looking modeling approaches by applying vehicle longitudinal dynamics and powertrain specifications. An intuitive graphical user interface has been designed to meet regulatory as well as research purposes for future updates by providing user-friendly operation capability. The model outputs have shown promising results, indicating that KEES will have sufficiently reasonable accuracy and fidelity to be used as an effective method for MHD commercial vehicle fuel efficiency regulations once well-measured and validated modeling inputs are provided. Some of the key issues presented and determined by the consortium in preparing the draft standards are also introduced to suggest the future direction of the legislation.