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 selectively connectable to the rotor, a second input shaft external to the first input shaft and fixedly connected to the rotor, an intermediate shaft parallel with the input shafts, first and second output shafts receiving torques from the first and second input shafts, and outputting a shifted torque based on the received torques, a planetary gear set mounted on the first output shaft, having a sun gear fixedly connected to the first output shaft, selectively receiving torques from the first and second input shafts and the intermediate shaft, and outputting a shifted torque based on the received torques to the first output shaft, and a plurality of gear sets allowing torque flows between the various shafts.