In this study, the pro-oxidant activities of 15 tea extracts were determined by the hydroethidine (HE) oxidation method. The results showed that order of the pro-oxidant activities of the tea extracts decreases in the order of black tea > oolong tea > green tea. Hence there is evidence that the pro-oxidant activity of the tea extracts at pH 7.4 increases with the degree of fermentation. Results indicated that theaflavins are better pro-oxidants than tea catechins. However, it was found that the total pro-oxidant activities of each tea sample correlates poorly with the sum of the weighted pro-oxidant activities of the phenolic compounds quantified in the tea extract. It was observed that the pro-oxidant activities of tea catechins were inhibited by superoxide dismutase (SOD), while theaflavins were not inhibited. A reaction scheme was proposed.