A series of Schiff base diphenylamines (SDs) have been successfully prepared in a one-pot synthesis process. These novel antioxidants exhibit higher antioxidative activities than the traditional antioxidants at elevated temperatures. In order to investigate the antioxidant mechanism, the reaction between antioxidant and peroxyl radicals is carried out. The isolated N-(4-(phenylamino)phenyl)benzamide, which is confirmed by the X-ray diffraction is suggested to be the key intermediate during the antioxidation process. Combined with the DFT calculations, the antioxidant mechanism is proposed that shows that SDs can be regarded as multifunctional antioxidants due to the radical-trapping ability of diphenylamine moiety and peroxyl radical-decomposing ability of imine moiety. Moreover, for practical application, the mono SDs exhibited extraordinarily higher antioxidant activities under the operating conditions of lubricant oil.