Electrical insulation properties of low density polyethylene (LDPE) are still unclear, especially when it is degraded. In this regard, ultraviolet (UV) photons were irradiated to LDPE sheets at room temperature and 85°C and their absorption spectra were measured at frequencies from 0.5 to 3.0 THz and from 18 to 105 THz. A very broad absorption band with its peak at around 1.6 THz appears by the UV irradiation. In accord with this appearance, the infrared absorption peak due to a carbonyl structure increases. Quantum chemical calculations were carried out using 3-dimensional models with and without carbonyl groups. As a result, the model with carbonyl groups was found to have optically active modes at about 1.7 THz. From these results, the increase in absorption at 1.6 THz is assumed to be induced by oxidation.