Abstract: A polyisobutylene base antioxidant having viscosity modifier function was synthesized using highly reactive polyisobutylene and 3-(3, 5-di-tert-butyl-4-hydroxy-phenyl) methyl acrylate as raw materials. The FT-IR analysis results showed that the structure of this synthesized antioxidant was consistent with that of the designed product. Results of TG analysis suggested that this antioxidant had excellent thermal oxidation stability and its incipient oxidative decomposition temperature was 40 oC higher than that of polyisobutylene. Rotary bomb oxidation test results showed this antioxidant exhibited good antioxidation properties, the oxidation induction time of mineral oil could be prolonged to 84 min (from 27 min) when this antioxidant was added with a mass fraction of 0.5%. The oxidation stability was improved evidently. When it was added with a mass fraction of 5.0%, testing kinematic viscosity of oil samples at 40 oC and 100 oC showed that the viscosity index of oil increased from 114 to 132, which indicated that this antioxidant also possessed the function of viscosity modifier.