After pre-metallization processing of AlGaN/GaN heterostructure with O2 plasma and HF solution, the Ni/Au Schottky contact characteristics were improved obviously and reverse leakage current reduced by three orders. In addition, annealing experiments were carried out at 200—600℃ for 5min in N2 atmosphere on many batches of Schottky diodes, the reverse leakage current decreased further with annealing temperature increasing. Especially after annealing at 600℃ for 5min in N2 atmosphere, the better uniformity of C-V characteristics at different frequencies indicated that surface trap density was reduced when Ni diffused to AlGaN/GaN surface during annealing. On the other hand, the C-V curves moved to the right and the reduction of absolute value of 2D electron gas depletion voltage proved that the Schottky barrier height was elevated when the annealing temperature increased.