Disclosed herein is a method of calculating the aging factor of a battery for a hybrid vehicle. The method includes the steps of detecting a first actual SOC value of the battery during operation of the vehicle; storing the first SOC value; initializing accumulated charge and discharge current amounts; determining whether the accumulated charge current amount is larger than a first predetermined value; determining whether the accumulated discharge current amount is larger than a second predetermined value if a charge current amount has not accumulated over the first predetermined value; detecting a second actual SOC value if a discharge current amount has accumulated over the second predetermined value; storing the second SOC value; calculating a theoretical accumulated current amount using the first and second SOC values; calculating an actual accumulated current amount using the accumulated charge and discharge current amounts; and calculating the aging factor of the battery using the theoretical and actual accumulated current amounts.