Abstract: The purpose of this paper was to solve the problem of eugenol on oxidation promotion at high concentration. The interaction of eugenol and syringaldehyde dimethylacetal(SGD)on their antioxidant activity was investigated using 2,2-diphenyl-1-picrylhydrazyl(DPPH)free radical scavenging method,isobolographic analysis,and ultraviolet spectrophotometer. The products of DPPH free radical scavenging reactions of eugenol,SGD and the combined antioxidants were measured by HPLC and high resolution mass spectrum. The results showed that the experimental IC₅₀ value point of seven tested combined antioxidants,eugenol and SGD with different ratios,were all located below the additive line and less than theoretical IC₅₀ value(P<0.05),and their interaction index λ of combination were less than 0.9,which depicted a significant synergistic antioxidant effect. In addition,the eugenol in combined antioxidant with optimum ratio 1:2 worked almost as twice eugenol used alone in the process of DPPH· scavenging reaction. The key product,4-((E)-3-(2,6-dimethoxy-4-(dimethoxymethyl)phenoxy)allylidene)-2-methoxycyclohexa-2,5-dienone,was speculated by analysing the HPLC chromatogram of products of DPPH free radical scavenging reactions and its high resolution mass spectrum. Consequently,the possible synergistic antioxidant mechanism of the combined antioxidants against DPPH· were deduced that the direction of reaction was quinonoids consuming.