Decline in petroleum reserves, increasing production costs and the sale of the gasoline, increasing greenhouse gases and global warming has led to methods for improving the performance of gasoline engines is proposed. The hybrid cars with low consumption and performance close to conventional vehicles are highly regarded. The selection process for a combustion engine of hybrid cars with the parallel powertrain is studied in this paper. In this study, the default target and the mission for the hybrid vehicle design to be examined. Furthermore, with the review of the domestic market, limits of the design and the construction would be determined. In this section, the performance of hybrid car engines with selected electric motors and each of combustion engines was simulated by the ADVISOR software, which could simulate the conventional cars or hybrid and hybrid-electric vehicles. After identifying the performance of the vehicle with any of engines, most important parameters for vehicle designing were selected, and by the Expert Choice software, the combustion engine for desired vehicle would be selected. The engine performance map, in order to plan for its compatibility with other components, were also prepared. Then, in the accordance with the mission, the proper electric motor was introduced. Finally, using the sensitivity in the engineering choosing process was expressed by calculating the sensitivity of important parameters in the design.