AlGaN/GaN high electron mobility transistors (HEMT) unpassivated with different gate lengths are irradiated with 60Co γ-rays to doses up to 1 Mrad(Si). The bigger the doses are and the smaller the gate lengthis, the greater the changes in drain current and transconductanceare. While the gate leakage current is significantly increased after irradiation, the threshold voltage is relatively unaffected. By analysing the series resistance of channel and the threshold voltage, we find that irradiation induced electronegative surface state charges is one of the important reasons of radiation damage.