GaAs power MESFETs with 0.8$mu$m gate lengths are fabricated using image reversal (IR) methods on the wafer with an undoped surface layer grown by MOCVD. The fabricated GaAs power MESFETs with an undoped surface layer show that an ideality factor 1.17, a built-in potential 0.83 V, a pinch-off voltage -2.7 V, a specfic contact resistance 1.21$ imes$10$^{5}$ ~3.42$ imes$10$^{2}$$Omega$-cm$^{2}$ and an extrinsic g$_{m}$ = 103.5 mS/mm. The maximum RF output power densities of the 0.8$mu$m devices are 360 mW/mm and 499 mW/mm, and power added efficiencies 29.67% and 29.05%, for the unit gate width 150$mu$m and 200$mu$m at 12 GHz.