The life span of a transformer usually depends on its insulation condition. In principle, materials used in transformer are subjected to ageing process during the operation that might lead to insulation failure. The interaction between transformer insulation with temperature, moisture and oxygen might commonly lead to the formation of gases, acid, sludge and moisture which accelerates the ageing process. Therefore, an appropriate oil treatment or purification process is required to delay the aging process by sustaining or improving the quality and properties of the insulation oil. Currently, in the field of oil treatment process, there are only a few studies done regarding the natural ester insulating oil (NEI) treatment compared to the mineral insulating oil (MI). In fact, there were no study done concerning the effect of temperature during treatment process toward NEI especially for the rapeseed and palm based oil. Thus, a drying treatment process was selected in studying the effect of temperature on NEI using a vacuum oven that have been set at two temperatures of 65° C and 90° Cwith 0.09MPa pressure. The experimentwas conducted for one week with a time interval of one day (24 hours). Quality of the treatment oil were then examined by evaluating its moisture content, acidity and AC breakdown voltage. In addition, this paper also investigated the temperature effects in determining the optimum time taken for oil treatment process. The finding of this work is believed to able in assisting the planning of an appropriate maintenance strategy to keep the power transformer oil in healthy condition, and hence met the standard requirement. © 2006-2018 Asian Research Publishing Network (ARPN).