Methyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl) propanoate (brand name T512) was loaded by mesoporous silica nanoparticles (MSNs) to afford the antioxidant for synthetic ester lubricant oil. Rotary oxygen bomb tests, pressure differential scanning calorimetry and oven oxidation tests were conducted to investigate the effect of the as-prepared antioxidant on the antioxidant stability of di-iso-octylsebacate (denoted as DIOS). When MSNs are combined with T512 at a properly selected mass ratio, the oxidation of the DIOS base oil is greatly slowed down while the oxidation induction time is significantly prolonged. This may be attributed to the gradual release of T512 from the MSNs and the adsorption of the oxidation products into the channels of MSNs.