Electron trapping after on-state stress and hole trapping after off-state breakdown stress have been observed by comparing the electroluminescence (EL) images and electrical characteristics in GaN high electron mobility transistors (HEMTs). Temperature measurement and two-dimensional device simulation have been done to confirm the hole-trapping phenomenon. With off-state breakdown stress, three devices all showed the same trend of electrical characteristic changes, a decrease of the threshold voltage and an increase of the gate leakage current. The changes of electrical characteristics were similar in different devices, but the changes in EL images varied. The technique promises a potential characterization tool for device and material screening.