Auto-manufacturers are under increasing pressure to develop powertrain systems for automotive vehicles, which are more efficient regarding fuel consumption, less polluting and still keep high performance levels. Hybrid electrical vehicles (HEV) are considered the most promising technology in sight, considering a time horizon of more ore less twenty years.HEVs combine benefits of electrical vehicles, such zero emission, low noise and high torques at low velocities and advantages of conventional vehicles, such as large autonomy, great reliability and high levels of performance. This paper is focused on the major elements of an HEV powertrain: electrical motors, internal combustion engine (ICE) and batteries, which are described. The paper also presents a comparison of two possible HEV configurations: series and parallel.The mathematical model of a small hybrid vehicle is developed using software ADVISOR. Series and parallel versions of this HEV are then compared and analyzed by numerical simulations, using the FTP-75 (Federal Test Procedure) a traffic cycle adopted by Brazilian regulation agency. Simulation results include emission levels of main automotive polluters, vehicle performance, state of charge of batteries during tests and overall efficiency of components.