The lubricating oil composition of the present invention comprising an ester mixture as the main ingredient consisting of (A) 60 - 95 % by weight of an ester of a hindered alcohol with a straight-chain saturated fatty acid having 8 - 12 carbon atoms, and (B) 5 - 40 % by weight of a complex ester of a hindered alcohol with a straight-chain saturated fatty acid having 8 - 12 carbon atoms and also with a dibasic acid having 2 - 50 carbon atoms, the composition having excellent performances such as biodegradability, high-temperature engine cleanliness, lubricity and piston seizure preventiveness which are conductive to environmental protection.