This study aims at studying thermal and stress breakdown of gallium nitride (GaN) high-electron-mobility transistors (HEMTs) with a high-power microwave (HPM) pulse injected. A thermal breakdown model, depending on its pulse width and duty cycle, is proposed at first, and an equation is derived for calculating power threshold when both geometrical and material parameters of transistor are known. A series of GaN HEMT-based power amplifiers is designed and measured by using one special HPM system for characterizing their power-to-failure values, and the calculated ones are then fitted for further determining failure temperature of GaN HEMT. Further, electrothermal simulation is performed for capturing temperature and stress distributions over the structure, where a few correlations are observed between the simulated and calculated results. Finally, the crack area in the field plate of GaN HEMT is also characterized experimentally.