Power transformers use mineral oil as an insulating liquid due to its outstanding dielectric properties. The poor biodegradability and toxicity of mineral oil have increased the interest in the use of a more environmentally friendly product such as ester-based oil. Generally, natural ester insulating oils (NEI) have a higher flash point and breakdown voltage compared to existing mineral oils. However, the higher price of NEI is the main obstacle to widely applied in power transformers. Therefore, alternative cheaper feedstock processing is required. This paper proposed waste cooking oil (WCO) as a potential alternative to the existing transformer insulating oil. The used of WCO promotes the optimal consumption of plant-based resources and more efficient waste management. Transesterification method is performed to remove the free fatty acids in the WCO and reduce the viscosity. The transesterification process is based on the chemical modification reaction between WCO, methyl alcohol (methanol) and sodium hydroxide (NaOH) catalyst lye that produces waste cooking oil methyl ester (WCOME). Chemical and electrical properties i.e. water content, acidity and breakdown voltage of the developed WCOME are compared with the existing WCO according to IEEE Guide for Acceptance and Maintenance of Natural Ester Fluids in Transformers (IEEE C57.147). © 2020 PENERBIT AKADEMIA BARU.