A power transmission system in a hybrid vehicle uses the power developed by an electric motor for directly rotating driving wheels, without power loss in a torque convertor connected to an engine. When the vehicle travels by the power of the engine, an input clutch 7 is engaged so that rotation of an engine output shaft 1a is transmitted to a two-gear shift device 9 via the torque convertor 6, the input clutch 7, and an intermediate shaft 21, whereby driving wheels 33a and 33b are driven by a differential unit 11. While powered by the electric motor, the input clutch 7 is released and the rotation of the electric motor 27 is preferably changed in the shift device 9 before being transmitted to the driving wheels 33a and 33b.