The antioxidative activity of a 70% methanol extract of sweet potatoes was estimated in a linoleic acid-aqueous system. The extract had a markedly strong antioxidative activity. Major phenolic components contained in the 70% methanol extract were identified as chlorogenic acid and isochlorogenic acid-1, -2 and -3 by using high-performance liquid chromatography. The other minor free phenolics were identified, or tentatively identified, as caffeic acid and 4-o-caffeoylquinic acid. Chlorogenic acid and/or isochlorogenic acids, however, had only slight antioxidative activity. From the results of the addition of chlorogenic acid, isochlorogenic acids and the other coexisting components contained in the sweet potato extract, the effective antioxidant activity of the sweet potato extract was proposed to be mainly based on the synergistic effect of phenolic compounds with amino acids.