A transmission system of a hybrid electric vehicle may include two shafts, two hollow shafts and two motor/generators. A first shaft connected to an engine. A first hollow shaft disposed without rotational interference with the first shaft, selectively connected to 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 to selectively transmit torque of the first shaft to the first hollow shaft. Clutches may connect the first shaft and the second shaft selectively 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.