A power transmission system of a hybrid electric vehicle includes an input shaft, a first planetary gear set having first, second, and third rotation elements, the second directly connected to the input shaft, and the third directly connected to a first motor/generator, a second planetary gear set having fourth, fifth, and sixth rotation elements, the fourth directly connected to the first rotation element and a second motor/generator, and the fifth selectively connected to the second rotation element, an output gear connected to the sixth rotation element, a first clutch selectively connecting the second rotation element with the fifth rotation element, a second clutch selectively connecting two rotation elements among the rotation elements of the first planetary gear set, a first brake interposed between the fifth rotation element and a transmission housing, and a second brake interposed between the third rotation element and the transmission housing.