Abstract: Different fuel diluted engine oils were prepared by adding different proportions 0^# diesel (5%, 10%, 15%, 20%) into CK-4 15W40 engine oil. Their physiochemical properties such as viscosity, pour point, flash point, total acid number and total base number were measured. Crankcase coking tester, pressure differential scanning calorimetry, optical interference point contact elastohydrodynamic lubrication tester as well as SRV tester were employed to investigate the coking, oxidation, elastohydrodynamic lubrication and boundary lubrication performances, respectively. Results show that viscosity and flash point experienced a drastic decrease with the increase of dilution rate and fuel dilution had a negligible effect on pour point. Total acid number, total base number and oxidation stability decreased to a certain degree. Film thickness of engine oil decreased with the increase of diesel concentration Friction reducing and anti-wear properties under boundary lubrication regime were also deteriorated with the addition of diesel.