A hybrid vehicle has an internal combustion engine, an electric motor, a transmission unit and driving wheels. The transmission unit includes a planetary gear unit and a one-way clutch, so that each output rotary speed of the internal combustion engine and the electric motor is changed. The planetary gear unit includes three elements: a ring gear, a sun gear and a carrier having at least one pinion. When the vehicle is in an under-drive cruising state, the first element of the planetary gear unit, which does not rotate, is connected to the output shaft of the internal combustion engine, the second element of the planetary gear unit, which rotates at a relatively high, speed, is connected to the output shaft of the electric motor, and the third element of the planetary gear unit, which rotates at relatively lower speed, is connected to the output shaft of the transmission unit. The one-way clutch is provided between the output shaft of the internal combustion engine and a case housing the transmission unit to allow the output shaft of the internal combustion engine to rotate in one direction only.