The aging behavior of transformer insulation Kraft paper aged in a natural ester oil, recently developed from palm kernel oil, is compared to the behavior of paper samples aged in mineral insulating oil. This study aimed to investigate the effect of the natural ester oil on the aging of cellulose insulation paper if used as an alternative insulating fluid in transformers. Thermally upgraded insulation paper was aged in both insulating fluids together with strips of galvanized steel, copper and aluminium to represent the transformer, under nitrogen pressure in a sealed mild steel pressure vessel for 150 degC for 28, 56, and 84 days. The degradation after aging was assessed using tensile strength and breakdown strength measurements of the paper. The paper samples impregnated with natural ester fluid exhibited similar behavior before and after aging, with an approximately 5% reduction in tensile strength after 28 days of ageing. Two sets of response were obtained for paper samples aged in mineral insulating oil under the same ageing conditions; the paper closer to the copper strips had a faster degradation rate (~35% reduction). The natural ester fluid retarded the aging of the Kraft paper. The characteristic breakdown strength of natural ester impregnated paper was found to be, on the average, 42% better than that of mineral oil impregnated paper.