A power transmission system of a vehicle may include: a first input shaft selectively connectable to an engine output through a first clutch; a second input shaft selectively connectable to the engine output through a second clutch; a first intermediate shaft; a second intermediate shaft; an output shaft disposed in parallel with and apart from the first intermediate shaft and transmitting power from the first intermediate shaft and the second intermediate shaft to a final reduction gear of a differential apparatus; a motor/generator fixedly connected to any one of the first input shaft and the first intermediate shaft; a continuously variable shifting device operably connecting the first input shaft to the first intermediate shaft; and a plurality of shifting gear sets operably connecting the second input shaft, the first and the second intermediate shaft and the output shaft with each other.