Described is a method for purifying long chain ester which is used as lubricant then. The purification method comprises: removing free fatty acids in the long chain ester by liquid-liquid extraction using alcohol, preferably ethanol or methanol, as solvent; wherein the total acid value of the purified long chain ester is less than 1mg KOH/g sample. Thereby, the purified long chain ester achieves the characteristics of oxidation stability which makes it suitable for being used as lubricant.