The changes in polyphenol oxidase activity in tea leaves during black tea manufacture were investigated. Polyphenol oxidase in the leaves was more active in summer than in spring and the tea clones for black tea had higher polyphenol oxidase activity compared with the clones for green tea. Enzyme activity in the leaves increased gradually in the lapse of the withering period, and the higher the withering temperature was, the faster the enzyme activity increased. The enzyme activity increased rapidly by rolling in both the high and low degree withered leaves, and reached about 2 to 3 times as much as that in the fresh leaves. Enzyme Activity decreased along with the fermentation process after rolling, and the higher the fermentation temperature was, the larger the decrease in activity was. It was presumed that the decrease in enzyme activity during the fermentation was caused by the formation of insoluble complexes of the oxidized polyphenols and the enzyme protein.