A power transmission system of a hybrid electric vehicle may include: an input shaft receiving torque of an engine; a first rotation shaft disposed in parallel with the input shaft; a first planetary gear set disposed on the first rotation shaft and including first, second and third rotation elements respectively connected to first motor/generator, output gear, and input shaft; a second planetary gear set disposed on the first rotation shaft and including fourth, fifth and sixth rotation elements respectively connected to second motor/generator and input shaft, second rotation element, and transmission housing; transfer gears forming externally-meshed gears; and frictional elements connecting one rotation element of the second planetary gear set to the input shaft, coupling the second planetary gear set, or connecting another rotation element of the second planetary gear set to the transmission housing.