Abstract: Effects of applying auxin(IAA) on enzyme activity and chemical components in tobacco after topping were investigated. The results showed that the nitrate reductase(NR) in the root and the leaves of flue-cured tobacco decreased, the invertase(INV) activity and root vitality were reducing tendency, the content of potassium increased and level of nicotine decreased, The reducing sugar and total sugar improved slightly, the starch decreased slightly, total nitrogen didn’t change much. Among the different concentration: 10 mg/kg, 20 mg/kg and 30 mg/kg, the 30 mg/kg was more suitable.