The influence of various nitrogen levels on the activeness of key enzymes of flue-cured tobacco leaves in carbon and nitrogen metabolism were studied. The results showed that as nitrogen level increased the amylase activity enhanced, the nitratase activity enhanced. In low nitrogen level, the time from nitrogen metabolism to carbon metabolism was shorten; however, in high nitrogen level, the time was deferred. In summary, the mid nitrogen level should be the optimum level to attain the highest yield and quality of tobacco.