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Chloroform and n-butanol extracts from the leaves of Dyera costulata exhibited EC50 values for DPPH radical scavenging activity of 79.8±0.2 and 12.0±0.1 μg/ml, respectively. Chemical investigation of the extracts was conducted by means of column chromatography. β-Amyrin and rhamnazin were isolated from the chloroform extract and quercetin-3-O-α-L-rhamnopyranoside was isolated from the n-butanol extract. The chemical structures of the isolated compounds were identified by comparison of their spectroscopic data with those reported in the literature. The EC50 value of quercetin-3-O-α-L-rhamnopyranoside in DPPH radical scavenging activity was 9.37±0.02 μM. Its antioxidative activity is therefore about 8 times higher than that of the well known antioxidant, BHT, which has an EC50 value of 80.78±0.01 μM.