The physical, chemical and electrical prosperities of MO is matched as an insulating material for transformer. Therefore, it can be used in transformers. Whereas, the MO is extracted from the fossil fuels. Due to vast consumption of fossil fuels, the accessibility of vestige fuel is going to run out near future. In addition, the MO (MO) is doesn’t meet the new environmental regulation due to it has less biodegradable. Therefore, this research proposed a new ecological friendly insulating oil for transformer which is renewable that of petroleum based insulating oil such as MO. Initially, a critical review has been made on recent development on alternate liquid dielectrics for transformer. Subsequently, critical characteristics of insulating oil have been measured according to the standard. Added to that, the critical characteristics of Natural Ester Oil (NEO) are estimated at diverse temperatures from 50°C, 70°C, 90°C, 110°C, 130°C, and 150°C respectively.