To amplify the antioxidant activity, we synthesized poly(catechin) by the enzyme-catalyzed oxidative coupling using horseradish peroxidase as a catalyst. The poly(catechin) showed great improvement in antioxidant activity such as radical scavenging activity against the superoxide anion and inhibition effects against free radical induced-oxidation of low-density lipoprotein, compared with a catechin monomer. In addition, poly(catechin) showed very high inhibition effects on xanthine oxidase activity, whereas the catechin monomer showed very less inhibition effects. The amplified activities might offer a high potential as a therapeutic agent for prevention of various free radicals and/or enzyme-related diseases.