This paper investigates breakdown voltages of enamel insulation of transformer wire in rice oil, corn oil and peanut oil. Experiments were performed with both new and aged conditions. It was found that vegetable oils show higher breakdown voltages in comparison with mineral oil for both new and aged conditions.Ageing process significantly affected the breakdown voltages of both vegetable oils and mineral oil. Enamel insulation in vegetable oils withstood higher breakdown voltages than in mineral oil. Ageing process had a negative effect on the breakdown voltage of enamel insulation. It greatly reduced the breakdown voltage of enamel insulation. It was also observed that enamel insulation markedly enhanced surface breakdown voltages of pressboard. Moreover, this enhancement is more pronounced in vegetable oils under aged condition.