The total contents of polyphenolics in potatoes measured by Folin Denis method were 42~76mg/100g in fresh weight. A major phenolic component contained in potato polyphenolics was identified as chlorogenic acid(3.6~15mg/100g in fresh weight). The antioxidative effects of potato polyphenolics and chlorogenic acid on the lipid peroxidation of liver microsome were studied in vivo and in vitro systems by measuring the formation of thiobarbituric acid reactive substances(TBARS) and the content of urine 8 hydroxy deoxyguanosine(OHdG). The TBARS contents of liver showed an increase in the cholesterol diet compared to those in the normal diet. This trend, however, was minimized when potato polyphenolics and chlorogenic acid were supplemented in the cholesterol diet. On the other hands, urinary 8 OHdG contents showed a marked increase with the supplementation of potato polyphenolics in the cholesterol diet. However, there was a trend of marked decrease by the supplementation of chlorogenic acid. In vitro study, potato poly phenolics and chlorogenic acid effectively inhibited the formation of TBARS in liver microsomal system in a dose dependent manners. These results suggest that potato polyphenolics exerts an antioxidative activity in cholesterol fed rat liver.