Due to toxicity and non-biodegradability of petroleum-derived mineral insulating oil, the use of vegetable-based oils such natural esters as insulating liquid is on grow. Although natural esters have higher flash point compared to mineral insulating oil, its high viscosity is not suitable for existing distribution transformer with natural cooling system. Thus, lowviscosity esters derived from various vegetable-based oils have been developed. In this study, waste cooking oil methyl ester (WCOME) is proposed as potential low-viscosity insulating fluid for transformer. Waste cooking oil (WCO) is cheaper vegetable-based oil relative to crude vegetable oil. It is also abundantly available as 50,000 tonnes were reported being produced in Malaysia each year. WCOME is produced via catalysed transesterification reaction between WCO and methanol usingpotassium hydroxide (KOH). The physical (density, flash point, pour point, viscosity), chemical (water content, acidity) and electrical (breakdown voltage) properties of WCOME are presented and discussed. Results indicated that transesterification reaction produced a low viscosity WCOME fluid (14.19 mm2/s) that possessed a dielectric breakdown voltage (BdV) of 30 kV, which is 50 above the IEEE C57.147 BdV's requirement for new natural ester fluids. Hence, the WCOME has a potential to be used as electrical insulating liquid for transformer. © 2006-2018 Asian Research Publishing Network (ARPN).