A power transmission system for a hybrid electric vehicle may include an input shaft to which a rotational power of an engine is input; a first motor/generator having a first motor shaft; a planetary gear set disposed on the first motor shaft of the first motor/generator and including a first rotation element fixedly connected to the first motor shaft, a second rotation element fixedly connected to the input shaft, and a third rotation element; a second motor/generator having a second motor shaft disposed; and an output shaft transmitting a rotational power transmitted from the planetary gear set and the second motor/generator to a differential apparatus, wherein the rotational driving force of the planetary gear set and the second motor/generator is transmitted to the output shaft by the first gear set and the rotational driving force of the output shaft is transmitted to the differential apparatus by the second gear set.