A T-gate forming method for a high electron mobility transistor includes the steps of: coating a first, a second and a third resist, each having an electron beam sensitivity different from each other, on a semiconductor substrate; performing a first exposure process by using an electron beam on the semiconductor substrate and then selectively developing the third resist; defining a gate head area by selectively developing the second resist to have a developed width wider than that of the third resist; performing a second exposure process by using an electron beam on the semiconductor substrate and then selectively developing the first resist in a bent shape at a temperature lower than in the development of the second and the third steps; and depositing metallic materials on the resists and then removing them to form a T-gate.