In this work, the long glass fiber reinforced poly(decamethylene terephthalanide) (PA10T/LGF) composites and short glass fiber reinforced PA10T (PA10T/SGF) composites were prepared by two different extrusion processing methods, respectively. The results of mechanical properties reveal the introduction of glass fiber can improve the performance of the material, and the LGF reinforced PA10T composites can achieve much higher performance than the SGF reinforced ones at the same level of fiber contents. The thermal stabilities of neat PA10T, PA10T/SGF, and PA10T/LGF are investigated by nonisothermal TG analysis. The thermal degradation kinetics is introduced by using Kissinger and Flynn-Wall-Ozawa methods and the thermal degradation mechanism functions of the samples are calculated by the Coats-Redfem model. The results show that the thermal stabilities of PA10T/SGF and neat PA10T are better than that of PA10T/LGF. In addition, the incorporation of glass fiber into PA10T do not have much effect on the reaction mechanism type of thermal degradation compared with that neat PA10T. Furthermore, the thermal stability analysis of the composites is verified by the computed results of Gibbs free energy (Delta G(#)). POLYM. ENG. SCI., 59:246-253, 2019. (c) 2018 Society of Plastics Engineers