Although plug-in hybrid electric vehicles (PHEV) are expected to reduce both consumption of petroleum and emission of CO₂ in comparison with ordinary hybrid electric vehicles (HEV), these advantages strongly depend on the reliability of batteries in PHEVs. For instance, their capacity reduction by degradation shortens the range of the travelling distance with electrical energy. However, there are no concrete test methods to evaluate the degradation of the batteries. In order to construct a test method for the evaluation of battery degradation, load conditions that reflect the situation of an actual vehicle in use has been investigated. In this study, the degrees of influence of two load terms, both a specific charge/discharge load (corresponding to the cycle life) and a thermal load (corresponding to the calendar life), in the capacity reduction were compared and a condition for an accelerated test was also discussed.