This study proposed a powertrain for BEVs that is capable of improving regenerative braking significantly by utilizing a lightweight flywheel and a small ultracapacitor pack as power sources. In order to connect the flywheel and ultracapacitor to the drive shaft effectively and more efficiently, a two-shaft electric motor called the transmotor is utilized. Further, the proposed powertrain, transmotor-flywheel assisted by ultracapacitor, enables us to reduce the power rating of the battery pack to the extent that it only be used for low power demand loads such as cruising.