This paper presents the propulsion system design of an internal-combustion hybrid electric vehicle. Hybrid electric vehicles are currently the most promising solutions for reducing fuel consumption and pollutant emission levels. The choice of suitable components is the key issue in the design procedure of a hybrid electric vehicle. Different selections and different sizing choices highly influence the overall performance expected from the vehicle. Having a solid understanding of the main components present in the hybrid electric propulsion system is the best way to predict the capabilities of the vehicle performance, fuel consumption and pollutant emission levels before building a prototype. Hence, sizing and selection criteria for the major components of the hybrid electric propulsion system are given here in order to configure the vehicle correctly. Finally, a case study is presented to demonstrate the propulsion system design procedure of a parallel hybrid electric vehicle.