The thermal degradation of eight types of high performance fibers (HPFs) was measured under nitrogen and air atmosphere. The degree of degradation, as measured by weight loss using thermogravimetric analysis (TGA), and the characteristic degradation temperatures were obtained. The kinetics of the thermal degradation has also been analyzed according to the Freeman-carroll method and the activation energies of the HPFs were estimated. The experimental results show that para-aramids (Kevlar (R) 29, 49, 129, and Twaron (R) 2000) have similar thermal stability, but their thermal degradation temperatures and activation energies in air are different from those in nitrogen, which means that the thermostability of the fiber depends not only on its intrinsic structure but also on the atmosphere and temperature of testing environment. Terlon (R) fiber shows higher degradation temperature as a copolymer of para-aramid, and its initial degradation temperature is 476.4 degrees C in air. It can also be found that the PBO (poly(p-phenylene benzobisoxazole)) fiber has the highest thermal degradation temperature among the samples tested, but its activation energy is not the highest in both air and nitrogen atmosphere. And the UHMW-PE (ultra high molecular weight polyethylene) fiber has the lowest thermal degradation temperature, and it begins to degrade when the temperature reaches 321.8 degrees C under air atmosphere. (c) 2005 Wiley Periodicals, Inc.