Many Chinese therapeutic herbs that are traditionally used in combination demonstrate significantly better pharmacological effects when used in the combination than when used alone. However, the pharmacological mechanism for this synergism is still not well understood. In the present study, the antioxidant activities of six herbs ((Paeonia lactiflora (PL), Atractylodes macrocephala (AMA), Angelica sinensis (AS), Astragalus membranaceus (AME), Glycyrrhiza uralensis (GU) and Rheum officinale (RO)), which were historically combined into eight traditional Chinese herb pairs (TCHPs) (AME-AS, AME-AMA, AME-RO, AME-GU, AME-PL, PL-AS, PL-AMA and PL-GU), were investigated in vitro by assessing the 1,1-diphenyl-2-picryl hydrazine (DPPH)-radical scavenging abilities of the herbs. The results of this study showed that all eight TCHPs had a significantly larger scavenging capacity than would be expected from the theoretical sum of those of the respective constituent herbs (p<0.05). Furthermore, the AME-GU, AME-PL and AME-AMA pairs not only showed a significant synergistic effect in the DPPH scavenging assay, but they also demonstrated similar results in hydroxyl radical and superoxide radical anion scavenging assays. Interestingly, the AME-AMA combination had a significantly higher superoxide anion (0.2 g/ml) and hydroxyl radical scavenging ability than the AME or AMA. The changes in the total phenolic and flavonoid contents were also investigated. Our study showed a significant correlation between the rate of enhancement in antioxidant capacity and the rate of increase in flavonoid content. Thus, the flavonoids are likely responsible for the synergistic effects present in TCHPs.