To study the changes in dielectric properties for thermally aged cross-linked polyethylene (XLPE) at different temperatures, the pure thermal and thermal-radiation aging tests are done. Thermal aging is carried at 80℃, 100℃, 135℃ and 155℃, lasting for 100 to 800 hours. The same conditions combined with 100 Gy/h gamma rays are done for thermal-radiation aged samples, and the dielectric spectra are measured from 25 to 200℃. The results show that the activation energy and values of β decrease obviously after the introduction of rays. At the same aging temperature and radiation conditions, electrical conductivity and relaxation peak frequency increase as the increasing aging time, which is a good reflection of dielectric properties during the aging process. Under different temperatures, the deviation in complex permittivity and modulus at lower frequency becomes more distinct compared with the values at a higher frequency. Finally, the dielectric properties under high temperature and low frequency could be a good indicator for the evaluation of XLPE degradation.