The AC breakdown properties of Kraft paper, pressboard and the enamel insulation layer in rice oil, peanut oil and corn oil were investigated under both new and aged conditions. The ageing of paper insulators and the enamel insulation layer in oils with the open cup method was carried out at 130 oC for 500 h in an air circulation oven. The puncture breakdown strength was determined by varying the number of layers and thickness of Kraft paper, while various electrode gap distances were used to measure the surface breakdown strength. Experimental results show that the breakdown strength and the tensile strength of paper insulators impregnated with these vegetable oils are comparable to that in mineral oil, and the surface breakdown strength of paper insulators is slightly affected by types of impregnating oils. The ageing significantly reduces both the breakdown strength and the tensile strength of paper insulators in investigated oils. In addition, it was also found that the enamel layer in vegetable oils withstood higher puncture breakdown strength than in mineral oil, and the ageing process significantly reduced the puncture breakdown strength of the enamel layer. This enamel layer was observed to markedly enhance the surface breakdown strength of pressboard.