In this article, the hot air aging of high strength glass fiber fabric/epoxy novolac resin (SW/EPN) composites was investigated by the aid of the aging behavior of EPN, mainly focusing on the microstructure evolvement of SW/EPN composites. The aging mechanism and thermal mechanical properties of SW/EPN composites were analyzed by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, thermo-gravimetric analyzer coupled with Fourier transform infrared spectrometry, and dynamic mechanical thermal analysis. The results showed that micro cracks initiated and propagated at the fiber–matrix interphase because of the heat and oxygen effect. After long-time aging at elevated temperatures, delamination phenomenon was discovered in SW/EPN composites. The results of weight changes showed that the degradation of EPN played a major role in SW/EPN composites. Moreover, the degradation of EPN contained post-curing, oxidation, and decomposition. The results also revealed that unaged EPN indicated two glass transition temperatures (Tg1 and Tg2). Tg1 increased for post curing while Tg2 decreased for oxidation with increasing of aging time and temperature. In the final period of aging at higher temperatures, only one Tg was observed because the formation of perfect crosslinked networks made EPN homogeneous. In addition, the relationship between Tg and chemical structure, as well as Tg and mass loss, confirmed that the variation of Tg depended on chemical changes. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40128.