A power transmission unit, which includes: an engine; a plurality of speed change gear pairs, each of which has a different gear ratio, and to which a power outputted from the engine is transmitted; an output member outputting the power transmitted from any of the selected speed change gear pair; a differential mechanism, which has three rotary elements to perform a differential action, and in which a first rotary element of the three rotary elements is connected with the engine; and an electric motor, connected with a second rotary element of the three rotary elements; and wherein the plurality of speed change gear pairs include a first gear pair adapted to be connected with the first rotary element and the output member, and a second gear pair adapted to be connected with a third rotary element of the three rotary elements and the output member. The power transmission unit can reduce a power loss, and is downsized to be mounted on a vehicle easily.