A transmission system of a hybrid electric vehicle may include first and second motor/generators, a planetary gear set, first and second output gears, and first and second clutches on an axis of an input shaft receiving torque of an engine in a transmission housing, a hollow shaft disposed at a radial exterior of the input shaft and adapted to transmit the torque of the engine or torque of the first motor/generator to the second output gear, an outer shaft disposed at a radial exterior of the hollow shaft and adapted to transmit torque of the second motor/generator to the first output gear, and a rear cover coupled to a rear end portion of the transmission housing, in which the planetary gear set includes a sun gear, a planet carrier and a ring gear as rotation elements thereof.