Phytochemical investigation of the crude methanolic extract of the leaves of Tapinanthus pentagonia (Loranthaceae) resulted in the isolation of four known flavonoids namely, quercetin 4’-methylether (1), 4’-methoxy-3’,5,7-trihydroxyflavone (2), quercetin-3-O-rhamnoside (3) and quercetin 3-O-rhamnoside4’- methylether (4). The structures of the isolated compounds were elucidated based on their 1D and 2D-NMR data. This is the first phytochemical study of that plant. The DPPH(2,2-diphenyl-1-picrylhydrazyl) radical scavenging activity, ABTS scavenging activity and ferric reducing antioxidant power (FRAP) were used to assess the antioxidant activities of the crude extract and three of the isolated compounds using catechin, ascorbic acid and gallic acid as standards, respectively. Results of the findings revealed that compound 3 exhibited the highest antioxidant activity in the three tests based on its IC50values followed by compound 4. Its IC50 value was 0.024 mg/ml comparable to that of catechin (0.015 mg/ml) for the radical scavenging activity. In summary, compounds 1, 3 and 4 exhibited good antioxidant properties and reducing power compared to the crude methanolic extract.