A power transmission apparatus may include a motor/generator having a rotor selectively connectable to an engine output shaft, a first input shaft mounted coaxial with the engine output shaft and fixedly connected to the rotor, a second input shaft external and selectively connectable to the first input shaft, first and second intermediate shafts and an output shaft mounted in parallel with input shafts, a planetary gear set mounted on the output shaft, having a sun gear fixedly connected to the output shaft, selectively receiving torques from the first and second input shafts and the first and second intermediate shafts, and outputting a shifted torque based on the received torques to the output shaft, and a plurality of external gear sets forming external engagements among the various shafts.