Thermal degradation behavior and mechanism of aromatic poly(1,3,4-oxadiazole)s fibers (POD fibers) under nitrogen and air were studied. The thermal stability of POD under air and nitrogen was investigated by thermogravimetric analysis, derivative thermogravimetry analysis and differential scanning calorimetry. The results reveal that in the initial stage of thermal decomposition, the thermal degradation behavior under air was the same as that under nitrogen, and in the end stage of thermal decomposition, oxygen had influence on thermal degradation behavior of POD. The changes of macromolecular structure of POD when heated at 500 A degrees C with different times were characterized by Fourier transform infrared, which illustrated that during thermal decomposition process some nitrile groups were formed, implying the disruption of oxadiazole rings. Subsequently, the whole macromolecular chain turned into carbonaceous materials at last. Thermogravimetric analysis-infrared spectra measurement further demonstrated that the carbon dioxide was the main volatilized product and the source of mass loss during thermal decomposition under nitrogen. Disruption of oxadiazole rings and formation of nitrile were further proved by pyrolysis-gas chromatogram-mass spectra characterization. The other characterizations, such as X-ray diffraction, scanning electron micrographs, were also applied to investigate the thermal degradation behavior of POD. According to these results, the mechanism of thermal degradation of POD was proposed.