Chemical modification was performed for improving the antioxidant activity of lead compound glycyrrhetinic acid (Ib). Two conjugated diene derivatives were prepared by reduction and dehydration reactions. Their in vitro antioxidant activities were studied using a cytochrome P450/NADPH reductase system from rat liver microsomes. The generation of microsomal free radicals was followed by oxidation of the DCFH-DA probe, while evaluating the capacity to inhibit reactive oxygen species (ROS) formation. The initial result showed that the two homo- and heterocyclic diene derivatives--18beta-olean-11,13(18)-diene-3beta, 30-diol (IV) and 18beta-olean-9 (11), 12-diene-3beta, 30-diol (V) exhibited strong antioxidant activities, at a concentration of 1.0 mg x mL(-1), they inhibited free radical (ROS) formation by 45% and 41%, respectively. In the same conditions, the lead compound (Ib) and the reference vitamin E inhibited ROS activity by 31% and 32%. Our results suggest that the elimination of the 11-keto group and the chemical reduction of 30-carboxylic group into hydroxyl function can increase the antioxidant activity of Ib significantly.