Tobacco enzymatic browning reaction induced by polyphenol oxidase(PPO) can influence the exterior color of tobacco leaves, lower interior quality and reduce economic benefits. In order to find out the change regularities of PPO activities in tobacco leaves at different positions at the key temperature point during the flue-curing process, the PPO activities of NC55 leaves at upper, middle and lower parts were determined in the study. The results were as follows. (1) In the whole flue-curing process, the PPO activity of middle leaves increased firstly and then decreased, while that of the lower and upper leaves decreased firstly, then increased and decreased again. (2) The PPO activity of leaves at lower and middle position reached the peak at about 46℃, while that of the upper leaves reached the second peak. The PPO activity almost disappeared after 54℃. (3) The PPO activity of the upper leaves was the highest before 38℃, while that of the middle leaves was the highest after 42℃. The study results could provide a certain guidance for optimizing the flue-curing technology of NC55.