We fabricated AlN/GaN metal insulator semiconductor field effect transistor (MISFET) and measured the DC characteristics of the MISFET when the device temperature was varied from room temperature to 500 degreesC. We systematically investigated the temperature dependence of the DC characteristics in the MISFET. It was found that the degradation of our device characteristics at high temperature is mainly caused by the increase of the gate leakage current due to the low barrier height at the gate Schottky barrier.