Abstract: The I-line photo-resist is used as resist exposed by both E-beam and optical system. It is exposed in beam system at 50KV, with dose between 50 100μC/cm² and developed for 1min. in 0.7% NaOH solution. The sensitivity of I-line photo-resist is 5 times than that of PMMA. Its resolution can get to 0.5μm. Two methods are employed to fabricate GaAs PHEMT. First, all of source and drain as well as gate was exposed by E-beam using I-line photo-resists. The GaAs PHEMT with 0.5μm gate length is fabricated by this way. Second, E-beam lithography is only used to define the fine part of source and drain as well as gate. All the other parts are patterned on a conventional contact optical system. The GaAs PHEMT with 0.25μm gate length is fabricated by this way.