The thermal decomposition behavior of the aged tetrahydrofuran copolymers with 3-azidomethyl-3’-methyloxetane (AMMO) or 3-nitratomethyl-3’-methyloxetane (NMMO) were studied by thermogravimetry and evolved gas analysis, and compared with aged polytetrahydrofuran (poly-THF). The thermal stability decreased in the order poly-THF > poly-AMMO-THF-AMMO > poly-NMMO-THF-NMMO, whereas the range of temperature for decomposition increased in the order poly-THF < poly-AMMO-THF-AMMO < poly-NMMO-THF-NMMO. The apparent activation energies of the degradation for polymers with ageing, calculated by the Ozawa method, were smaller than that without ageing.