A power transmission system of a hybrid electric vehicle may include an input shaft receiving torque of an engine, an output gear, a first planetary gear set disposed on the input shaft and including a first rotation element connected to a first motor/generator, a second rotation element, and a third rotation element connected to the input shaft, a second planetary gear set disposed on the input shaft and including a fourth rotation element connected to a second motor/generator, a fifth rotation element directly connected to the second rotation element and to the output gear, and a sixth rotation element selectively connected to a transmission housing and selectively connected to the first rotation element, and frictional elements selectively connecting two rotation elements of the second planetary gear set, selectively connecting the first rotation element to the sixth rotation element, or selectively connecting the sixth rotation element to the transmission housing.