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• Bio-degradable vegetable-based oils are 3rd generation insulating fluids in order to replace conventional mineral oil. • Dispersion of TiO 2 and ZnO results in enhancement of dielectric properties of vegetable oil based oil-blends. • Detailed statistical analysis of Weibull data regarding the failure probabilities confirms the experimental results. • TiO 2 and ZnO dispersed vegetable oils can be used as insulating fluids to improve the rating of transformers. Mineral oil being the most extensively used oil in transformers still has some associated drawbacks like non-biodegradability and low fire point. Bio-degradable, non-toxic and fire-resistant vegetable-based ester oils are considered as second-generation insulating fluids. These oils are continuously being tested in order to replace the conventional mineral oil for insulation purposes. The dielectric properties of these vegetable oils are further improved by adding various types of nanomaterials. This paper focuses on preparing a mixture of natural ester oil and mineral oil blend as a base fluid and studying the dielectric behavior of this fluid after being dispersed with TiO 2 and ZnO nanoparticles. Results indicated that dispersion of both nanomaterials resulted in an overall enhancement in the dielectric properties of nanofluids. The breakdown voltage of TiO 2 dispersed nanofluid is reported to be about 30% greater than pure mineral oil, 24% greater than pure ester oil, and 17% greater than the oil-blend. Likewise, the breakdown voltage of ZnO dispersed nanofluid is observed to be around 28% greater than pure mineral oil, 19% greater than pure ester oil, and 16% greater than the oil-blend. Moreover, in terms of cooling, TiO 2 dispersed oil-blend shows better performance as a coolant. Thus, both types of nanofluids can be used as insulating oils in the transformer depending upon the requirements expected to be met by the transformers. [ABSTRACT FROM AUTHOR]