The activities of polyphenol oxidase and peroxidase in three flue-cured tobacco varieties during aging were analysed and the changes of the zymograms of the two enzymes' isozymes were studied parallelly with electrophoresis. The results showed that the activities of polyphenol oxidase and peroxidase increased first and then decreased in the course of aging; the zymograms of the isozymes of polyphenol oxidase and peroxidase changed relatively remarkably during aging. It demonstrated that: during the first three months of aging, only a few dim isozyme bands were presented; the amount and brightness of isozyme bands peaked in the 6th month of aging, and diminished gradually afterwards. The analysis of the zymograms of the isozymes verified the rule of activity change of oxidase and peroxidase in tobacco leaf during aging.