The degradation behavior of cross-linked polyethylene (XLPE) exposed to heat and radiation was investigated, focusing on its changes in chemical structure and mechanical properties. Infrared absorption spectroscopy (FT-IR), differential scanning calorimetry (DSC), and tensile tests were conducted. At the aging condition becomes more severe, the absorption due to carbonyl groups increases. In accord with this, the elongation at break decreases exponentially, while the enthalpy of fusion due to the melting of crystals decreases. When XLPE is exposed to heat and radiation, it is oxidized, which lowers the crystallinity and makes the crystals smaller. As a result, the mechanical properties of XLPE become degraded.