Under different stress, the current collapse, gate current leakage and breakdown voltages of AlGaN/GaN high electron mobility transistors change before and after high temperature annealing. The results show that characteristics of devices are greatly improved for AlGaN/GaN high electron mobility transistor after Schottky high temperature annealing. Interface of Schottky contacts is studied by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) before and after high temperature annealing. The analysis indicates that eliminating the medium between Ni and AlGaN and reducing of traps near the surface of AlGaN can improve the effective Schottky barrier, which can enhance the electric characteristics of the devices.