PURPOSE: To obtain a lubricating oil composition capable of keeping excellent defoaming property over a long period and suitable as engine oil, gear oil, hydraulic fluid, automatic transmission oil. bearing oil, etc., by compounding a specific amount of a perfluoroether as an essential component.

CONSTITUTION: The objective composition is produced by using a perfluoroether preferably expressed by formula [R1 and R2 are 1-4C perfluoroalkyl; R3 is 1-4C perfluoroalkylene; (n) is integer of ≥2 (preferably 10-300)] as an essential component and adding 1-1.000wt.ppm of the perfluoroether to a synthetic or mineral base oil having a viscosity range of preferably 2-1,000cSt (at 40°C).