Abstract: Effect of treatment by enzymes on the chemical quality of flue-cured tobacco leaves was studied. Leaves of YN99, the major tobacco variety cultivated in Mt. Qinling area, were used in the experimental treatments with 7 enzymes, including a neutral protease, papain, flavor protease, cellulase, pectinase, glucoamylase, and α-amylase, at 6 concentration levels. The results showed that the treatments effectively increased the sugar content, but decreased the total alkaloid, nitrogen and starch contents in the tobacco leaves. A grey correlation analysis indicated the 10 treatments that produced the greatest improvements on the tobacco chemical quality to be T5A6, T5A5, T1A2, T5A2, T5A3, T6A5, T6A4, T1A3, T6A3, and T5A4, in that order. The tobacco leaves treated by 100 and 120 U·g^-1 pectinase showed the most desirable and balanced result; whereas, those were exposed to the flavor protease or α-amylase did not fare well in all regards. A cluster analysis on the 42 treatment samples classified them into 4 categories showing similar chemical qualities when treated by a same enzyme.