Two kinds of bis-phenol epoxy resins, a soft one (SE) and a hard one (HE), were aged thermally with and without simultaneous irradiation of gamma rays. For each resin, glass transition temperature (Tg), thermal decomposition temperature, elongation at break, tensile strength, and flexural modulus were measured. The difference in Tg between SE and HE is significant, which exerts certain influences on their degradation properties. When aged, SE becomes hard and it is crosslinked, regardless of whether the simultaneous irradiation was given or not. Finally, SE becomes brittle. In contrast, the mechanical properties of HE are scarcely affected by the two aging processes. However, HE finally becomes hard if aged thermally, while it is decomposed by the irradiation.