A power transmission system of a hybrid electric vehicle may include a first input device, a torque transmitting device, a second input device, an output device, and a final reduction device. The first input device may receive either or both of torques of an engine and a first motor/generator. The torque transmitting device may be disposed at a rear or downstream of the first input device and selectively receive torque from the first input device. The second input device may be disposed at a rear or downstream of the torque transmitting device, convert torque of a second motor/generator, and output the converted torque. The output device may receive torque from the torque transmitting device and/or the second input device and output the torque. The final reduction device may output the torque transmitted from the output device as driving torque.