The balance between the motor driving power and the road driving resistance power was paid more attention in respect of the fuel economy of engine for range extended vehicle. Based on this feature, a special engine was developed for range extended electric vehicle. The engine adopted a high expansion ratio design based on the Atkinson cycle and could provide the power generation output of 35 kW. After matching the electric vehicle, the comparison of vehicle driving economy at constant speed was conducted between the Otto cycle engine and Atkinson cycle engine range extender based on the same power. The results show that the engine for range extender of Atkinson cycle has higher power generation efficiency with a wider range of vehicle type.