Abstract: urnout characteristics of C band GaAs power MESFET are studied by-means of Scanning Auger Microprobe(SAM),Secondary Electron Microscope(SEM),Electron Probe Microarea Analysis (EPMA),X-ray Transmission Microscope,etc.in this paper. The burn out modes are suggested and discussed with the presentation of the associate distribution graph of failure rate and their surface topography. The results indicate that the source-drain burnout is the primary failure mode ill GaAs power BET, with the other failure modes including the“air-bridge”burnout,heat-spot burnout, and that caused by“thorn”at drain or source edge, defect and contamination“particle”in the chip and the imperfect during the process of material and device. The failure mechanisms are also discussed.