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