The aims of this study were to evaluate the antioxidative effect of a traditional Taiwanese vegetable, Houttugnia cordata Thunb. (H, cordata), by subjecting rodents to oxidized frying oil-induced oxidative stress, and to examine the antimutagenic effects of H. cordata using the Ames test. Forty-eight Sprague-Dawley rats were fed a diet of 0, 2, or 5% H. cordata and 15% fresh oil or oxidized frying oil (OFO) for 28 d. Levels of polyphenol in the feces, plasma, and liver were determined. The LDL lag time, plasma total antioxidant status (TAS), and levels of thiobarbituric acid-reactive substances (TBARS) were used as antioxidative indices, and the protein carbonyl group was used as an oxidative index. The results showed that the polyphenol content decreased in the plasma and increased in the feces when administering OFO, and the apparent absorption of polyphenol also decreased. The polyphenol content in plasma increased when giving H. cordata. There was a higher polyphenol concentration in the water extracts of H. cordata than in the methanol extracts. The OFO-fed groups had higher plasma TBARS and hepatic protein carbonyl group concentrations and shorter LDL lag times than those of the control group. The total TAS was elevated and the LDL lag time was prolonged when fed with H. cordata. In addition, both water and methanol extracts of H. cordata had an antimutagenic effect on benzo(a)pyrene, aflatoxin B₁, and OFO, and showed a dose-dependent response using the Ames test. The antimutagenic ability of water extracts was higher than that of the methanol extracts. In conclusion, the polyphenol in H. cordata is easily absorbed and metabolized by rodents. H. cordata showed both antioxidative and antimutagenic properties under OFO feeding-induced oxidative stress.