Improvements have made on the fuel efficiency and emission control of passenger cars, and the development of the hybridization of vehicles has been further assisting improvements. But great technological developments are required to see similar improvements to heavy-duty vehicles (HDV), including large city-buses and heavy-duty trucks, and the hybridization of vehicles is a vital method for resolving this problem. However, technology has not yet been developed far enough to achieve results in which performance is cost effective. As the mileage of HDV over their whole life cycle is approximately 10 times longer than that of passenger cars, designing a new system for HDV requires a different perspective. This paper clarifies the current problems occurring in the hybridization of HDV in order to improve fuel efficiency. This paper also proposes the effectiveness of installing a Fly Wheel system as a resolution for these problems.