The significant use of petroleum-based product has hitherto been justified by its wide availability, good properties, good combination with cellulose and low-cost. However, with environmental issues now becoming extremely fundamental, the need for a product with a high fire point temperature and high biodegradability has become imperative. The discovery of natural ester fluids based on renewably sourced plants and vegetable oils have paved ways for the new insulating fluids for use in transformers and power equipments. The efficacy attributed to the various types of vegetation-based insulating fluids in many industrial situations have made them an indispensable ingredients of industrialization globally and development that has characterized the past century. This review presents a comparison of the fundamental thermal, insulating and coolant properties of mineral oil, synthetic esters and natural esters.