The thermal degradation behavior of meta- and para-aramid fibers (PMIA and PPTA) in different atmospheres was studied by thermogravimetric analysis coupled with mass spectrometry (TGA-DTA/MS). The process of mass loss in the degradation of PMIA was divided into three steps in both argon and air. The mass loss of PPTA was greater than that of PMIA in argon when the temperature reached 1000 degrees C. Simultaneous TG/DTA measurements showed that the thermal degradation was endothermic in argon and exothermic in air for both fibers. Mass spectra of volatiles were obtained, and PMIA and PPTA gave similar mass signals. The temperatures of maximum abundances in ion currents corresponded to the temperatures Of mass loss peaks in the DTG curves. The relative values of ion currents were compared at various temperatures. The yield of CO2 was the largest fraction in ion currents of all the volatiles, and the abundances of ion currents of phenyl compounds were low. The elemental analyses of decomposed PMIA and PPTA fibers provided direct information of chemical and morphological changes. A synthetic kinetic model was introduced, and the analyses of kinetic parameters were used to explain the degradation progresses.