A power transmission system of a hybrid electric vehicle includes an input shaft configured to receive torque of an engine. A first motor/generator includes a first rotor directly connected to the input shaft and a first stator selectively connected to a first motor housing and the input shaft. An intermediate shaft is disposed in series with the input shaft and selectively connected to the first stator. A second motor/generator includes a second rotor directly connected to the intermediate shaft and a second stator directly connected to a second motor housing. A final reduction gear is configured to receive torque from the intermediate shaft and to output the torque. A battery supplies electricity to the first and second motor/generators or is charged by torque of the first and second motor/generators.