Dried flower extracts of Hibiscus sabdariffa L., a local soft drink material and medical herb, was found to possess antioxidant activity in the present study. In the preliminary studies, antioxidant potential of three fractions of the ethanol crude extract (HS-C: chloroform-soluble fraction; HS-E: ethyl acetate soluble fraction; HS-R: residual fraction) obtained from the dried flowers of Hibiscus sabdariffa L. were evaluated by their capacity of quenching 1,1 -diphenyl-2-picrylhydrazyl (DPPH) free radical and inhibiting xanthine oxidase (XO) activity. HS-E showed the greatest capacity of scavenging free radical (EC50=0.017mg/ml), and HS-C showed the strongest inhibitory effect on XO activity (EC5o=0.742 mg/ml). Furthermore, antioxidant bioactivities of these crude extracts were investigated using a model of tert-butyl hydroperoxide (t-BHP)-induced oxidative damage in rat primary hepatocytes. All fractions were found to inhibit significantly the unscheduled DNA synthesis (UDS) induced by t-BHP at a concentration of 0.20 mg/ml. HS-C and HS-E also decreased the leakage of lactate dehydrogenase (LDH) and the formation of malondialdehyde (MDA) induced by t-BHP (1.5 mM) considerably at a concentration of 0.10 and 0.20 mg/ml in the rat primary hepatocyte cultures. These results indicated that the dried flower extracts (HS-C and HS-E) of H. sabdariffa L. protect rat hepatocytes from t-BHP-induced cytotoxicity and genotoxicity by different mechanisms.