Flame retardant ethylene-propylene-diene rubber (FR-EPR) and silicone rubber (SiR) are two major materials for safety-related cables in nuclear power plants. For such cables, degradation of their insulation is a matter of serious concern. Regarding the above, sheets of FR-EPR and SiR were aged either thermally or concurrently by heat and gamma radiation and the resultant changes in various properties were systematically compared and analysed from various aspects by FT-IR, differential scanning calorimetry (DSC), thermogravimetry/differential thermal analysis (TG/DTA), and scanning probe microscopy (SPM), and by measuring indenter modulus, elongation at break (EAB), tensile strength (TS), and complex permittivity. As a result of the experiments, both FR-EPR and SiR become hard in a similar manner in both cases where the degradation was given by heat only or concurrently by heat and radiation. In both polymers, degradation induced cross-linking and breaking of chemical bonds were observed by FT-IR, DSC, TG/DTA. Furthermore, a contrastive difference in dielectric behavior between SiR and FR-EPR was also observed in complex permittivity.