Breakdown voltage is an important indicator in determining whether the quality of the isolation of an insulator is good or bad, including liquid insulation. Breakdown voltage of liquid insulation is different for each different type of oil. There are several things that influences the decline in value of breakdown voltage and insulation qualities of this liquid insulator and some of which are thetemperature rise and age of the transformer oil. The rising temperatures of the transformer oil is happened because the transformer was used in high load conditions continously. The age of transformer oil is calculated based on the length of the oil it self is used in the transformer. Testing the effect of temperature rise and the transformer oil’s age to breakdown voltage is necessary to know the degradation of transformer oil breakdown voltage as one indicator of the merits of a power transformer liquid insulation.