A power transmission system of a hybrid electric vehicle which uses an engine and first and second motor/generators as power sources may include an input device having a first input shaft receiving either or both of torques of the engine and the first motor/generator and provided with a first driving gear disposed thereon, and a second input shaft disposed in parallel with the first input shaft, receiving a torque of the second motor/generator, and provided with a second driving gear disposed thereon, an output device including an output shaft provided with one driven gear engaged with the first driving gear, a second driven gear engaged with the second driving gear, and an output gear disposed thereon, and outputting torque transmitted from the input device, and a final reduction device outputting torque transmitted from the output device as driving torque.