In this paper, performance degradations of enhancement mode(E-mode) gallium nitride(GaN) high-electron-mobility-transistors(HEMTs) under accelerated power cycling tests are presented. For this purpose, a DC power cycling setup is designed to accelerate the aging process in a realistic manner. In order to evaluate the aging-related parameter shifts and corresponding precursors, electrical parameters are periodically monitored through a high-end curve tracer. Both the cascode device and p-GaN gate GaN devices are evaluated during these tests. In the experimental results, it is observed that the on-state resistance gradually increases in both devices. Meanwhile, the threshold voltage of the p-GaN gate GaN device gradually increases over the aging cycles and a remarkable variation in the transfer characteristics is observed. At the end of the tests, failure analyses are conducted on both devices. The cascode GaN devices show both short and open circuit failure modes, and a weak point in the drain-side bond wires is detected. For the p-GaN gate GaN device, the electrical parameter shifts indicate a possible gate degradation after the device is aged.