A transmission system of a hybrid electric vehicle may include a first shaft connected to an engine, a first hollow shaft disposed without rotational interference with the first shaft and provided with a first output gear, a second hollow shaft disposed without rotational interference with the first hollow shaft and provided with a second output gear, a second shaft disposed in parallel with the first shaft and operably connected to the first shaft and the first hollow shaft through a gear unit so as to selectively transmit torque of the first shaft to the first hollow shaft, a clutch selectively connecting the second shaft to the first hollow shaft, a first motor/generator operably connected to the second hollow shaft, and a second motor/generator operably connected to the second shaft.