A power transmission system of a hybrid electric vehicle may include an input shaft, an output shaft, a first planetary gear set having three rotation elements, a second planetary gear set having three rotation elements, a third planetary gear set, a first rotational shaft, a second rotational shaft, a third rotational shaft, a fourth rotational shaft, a fifth rotational shaft, a sixth rotational shaft, and a seventh rotational shaft such that the power transmission system can suppress conversion to an ENG mode by providing sufficient power performance at the time of wide open throttle (WOT) launching and maximally use power of an engine at the time of conversion to a first HEV mode and a third HEV mode.