An optimization tool to identify optimum structure/parameters of hybrid electric vehicles (hereafter abbreviated as HEV) is proposed, which can take account of various criteria regarding environmental impacts, cost minimization, or minimum fuel consumption. The scope of hybrid vehicles is extended to include normal ICE to dual mode vehicles using electric power train. In the paper, general framework and concepts of the study, namely framework of simulation model and optimization algorithm, are introduced briefly with its background activities. With this tool, preliminary evaluation was made to understand possibility of improvement of energy efficiency by those alternative vehicles, and some of currently obtained results are shown which is still under development and investigation.