Disclosed herein is a technique for determining degradation of a high-voltage battery of a vehicle. More specifically, variations in charge capacity with respect to a predetermined variation in voltage of a high-voltage battery that is currently being slowly charged are obtained depending on degradation degrees of the high-voltage battery. A vehicle equipped with a high-voltage battery having a specification identical to those of the high-voltage battery is used to acquire a charge capacity and a voltage of the high-voltage battery that is currently being slowly charged. A variation in charge capacity with respect to the predetermined variation in voltage is calculated from the charge capacity and the voltage. The variation in charge capacity with respect to the predetermined variation in voltage is compared with the variations in charge capacity with respect to the predetermined variation in voltage depending on the degradation degrees, thus determining a relevant degradation degree.