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This paper presents a hybrid controller for hybrid electric vehicles using hybrid system theory. The hierarchical control structure of parallel hybrid electric vehicle (HEV) is introduced first and the drive control problem of the vehicle is formulated using hybrid system theory. State machine theory is applied to solve the problem of vehicle operating mode transitions. Then a fuzzy torque distributor is developed to deal with power splitting problem between the engine and the motor so that these power sources are operated at high efficiency. A backward-looking parallel HEV model was used for implementation and simulation of the controller. Potential fuel economy improvement has been shown by using this hybrid controller.