A composite power transmission mechanism 10, comprising the combination of a planetary gear unit 15 and a differential planetary gear mechanism 16, wherein a planetary gear is connected with a drive shaft of a vehicle, while other three gears let the shafts of the motor 12, a motor 13 and an engine 11 rotate respectively. The drive shaft of the vehicle is made to rotate by means of the planetary gear of the composite power transmission mechanism; since the rotation of different sources of a driving force, i.e., those of the motor 12, the motor 13 and the engine 11, can be transmitted to one another by only one meshing of the gears, the mechanical loss that is dependent on the number of times of the meshing of the gears can be reduced to a minimum.