To obtain novel structural antioxidants that have different antioxidant mechanisms, four 2-(alkylthio)-N-(4-(phenylamino)phenyl)acetamides 2a–d as dual functional antioxidants are designed, synthesized, and confirmed by 1H-NMR, FTIR, MS, and elemental analysis. The antioxidant behavior of compounds 2a–d as additives of base oil triisodecyl trimellitate (TIDTM) is evaluated by non-isothermal and isothermal DSC analyses. The results showed all compounds can greatly increase the incipient oxidation temperature (IOT) and oxidation induction time (OIT) of TIDTM, especially, compound 2c exhibited an OIT value of 72.5 min at 230 °C, which is almost 28 times the length of TIDTM. Moreover, compounds 2a–d do not affect the tribological performance of TIDTM. The mechanism of antioxidants involved an intramolecular synergism are proposed. This work demonstrates compound 2c can be used as a novel potential antioxidant additive of TIDTM; in addition, it would inspire the emergence of highly potent antioxidants with different antioxidant mechanisms.