To achieve higher efficiency of HEVs, the optimization of each size and weight of power train components becomes more important for various vehicle segments because its system is more complex rather than conventional ICE vehicles. To solve this problem, models of a Diesel ICE, a motor and a battery are constructed for various sizes and weights. These models are used for optimization of hybridization of power train under NEDC mode conditions. The optimization algorithm is also used for optimization process. As a result, the optimum combination of the components for each vehicle segment is estimated.