In this study, the antioxidant property of (+)-catechin-aldehyde polycondensates has been examined. Superoxide anions are one of the most typical reactive oxygen species (ROS) and generated by xanthine oxidase (XO). The measurements of the superoxide anion scavenging and XO inhibition activity showed that catechin had pro-oxidant properties in lower concentrations and little XO inhibition. On the other hand, the polycondensates exhibited much higher effects compared to the catechin monomer, and their physiological activities were greatly affected by the structure of polycondensates. Steady-state analysis of the inhibition against XO showed that the inhibition type of the polycondensate was uncompetitive. Furthermore, the results of the circular dichroism and UV-visible measurements of a mixture of the polycondensate and XO were in good agreement with that of the steady-state analysis; the spectral changes due to the chelation of the polycondensate onto the Fe/S and/or the FAD center of XO were observed. These data strongly suggest that the polycondensates possess a great potential as antioxidant for various applications.