Chemically cross-linked polyethylene irradiated by γ-rays to certain designated doses from 100 to 800 kGy at 100°C in air was analyzed by X-ray diffraction, infrared spectroscopy and terahertz spectroscopy. It was found by infrared spectroscopy that the degree of oxidation increased when the sample was irradiated. By the terahertz spectroscopy, a peak appears at around 44 cm -1 (≃1.3 THz) when the sample was irradiated by γ-rays. By carrying out the Cole-Cole analysis on the complex permittivity spectra associated with the peak at 44 cm-1, the dipolar relaxation was found to exhibit a nearly single relaxation process. This suggests that the terahertz spectroscopy has a potential to detect the oxidative degradation of cross-linked polyethylene.