Fuel consumption of a hybrid electrical vehicle (HEV) depends on the driving cycle (i.e. the use of the vehicle), on the powertrain components and on the energy management. The paper proposes an optimal component sizing process taking into account the entire system. An optimal energy management is used in order to minimize the fuel consumption on a driving cycle. This optimization approach of the HEV uses Radial Basis Function models which allow a fast constrained optimization.