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Sheets of flame‐retardant cross‐linked polyethylene (FR‐XLPE) were aged thermally at 80, 100, 135, and 155 °C for various periods from 100 to 800 h. They were also aged concurrently by heat and gamma rays at the same temperatures for the same periods at a dose rate of 100 Gy/h. For these sheets, we measured indenter modulus, elongation at break, complex permittivity, and absorption spectra at midinfrared and THz frequencies. As a result, it has become clear that the aging at 135 °C makes FR‐XLPE hard regardless of the presence of concurrent irradiation of gamma rays. This makes the transport of charge carriers difficult. Therefore, both the real and the imaginary parts of complex permittivity decrease, which is significant if the measurement frequency is low. However, this tendency becomes opposite as the samples were aged at 155 °C by heat or concurrently by heat and radiation. Using indenter modulus as an indicator, it has become clear that the aging progress is composed of two different stages, which could be a good clue to the explanation of these opposite changes in complex permittivity. On the other hand, the samples aged at 155 °C concurrently by heat and radiation show larger values in the indenter modulus and in two parts of complex permittivity than the samples aged at the same temperature only by heat. This is probably due to the oxidative degradation induced by the cooperative reaction of heat and radiation. © 2019 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc. [ABSTRACT FROM AUTHOR]