Relative thermal stability of polyimides has been investigated by TG．In order to evaluate roughly the heat resistance of polyimides from the data of relative thermal stability according to the mechanism of oxidation-dehydrogenation，we tried to establish a quantitative relationship between chain structure factor and polyimides has been pro-posed as follows：where thermal index T20．000 is the temperature，at which the material can be used for life time 20，000 hrs；KH is the hydrogen fraction，which equals to number of hydrogen atoms divided by the number of total atoms in repeating unit of the macromolecule；a，b are constants．The kinetics of thermal and thermo-oxidative degradation has been discussed．Apparent activation energy of thermo-oxidative degradation of polyimides is not cons-rant throughout the whole process，but decreases as the reaction proceeds．The activa-tion energy of thermal degradation remains constant．