Since the introduction of feed-in tariff programs, the construction of large-scale photovoltaic (PV) power plants is rapidly increasing in Japan. Although the minimum required lifetime of PV systems is 20 years, some PV panels show a reduced capacity to generate electric power over the long term, and at present the reduction in performance can be evaluated only by measuring their generating capacity. One possible cause of this reduction is considered to be degradation of the polymer material used for the PV backsheet. In this study, the commercial PET film most commonly used for the PV backsheet was evaluated. The rate of degradation of PET film was investigated by measuring its mechanical properties and partial discharge characteristics. It was found that the rate of degradation of PET film could be effectively evaluated from the partial discharge characteristics.