A power transmission apparatus may include a first input shaft, a second input shaft external and selectively connectable to the first input shaft, a third input shaft external to the second input shaft and selectively connectable to the first input shaft, an intermediate shaft, first and second output shafts, first and second idle shafts, a planetary gear set mounted on the intermediate shaft and having first to elements and where two elements are selectively connectable to the intermediate shaft, a plurality of gear sets allowing torque flows between the various shafts and the planetary gear set, first and second synchronizers respectively mounted on the first and second output shafts, selectively receiving a torque from the second and third input shafts through two paths, and transmitting the received torque to the first and second output shaft, and four clutches.