Abstract: In order to explore the transformation process of free amino acids in tobacco leaves with different maturity during the flue-curing process, we used flue-cured tobacco K326 in studying the content of free amino acids and transaminase activities. During the curing process of tobacco leaves, the contents of 10 species of amino acids such as glutamic acid and the total amount of amino acids showed significant changes. The differences in the contents of glutamic acid, alanine, phenylalanine, proline and aspartic acid in different maturity tobacco leaves were mainly observed at the late yellowing stage and the early leaf-drying stage. During these two stages of curing, proper-mature and over-mature tobacco leaves had higher transaminase activity. Compared with immature tobacco leaves, the contents of serine, histidine, arginine and lysine in the proper-mature and over-mature tobacco leaves showed a decreasing change starting from the leaf-drying stage, indicating that the maturity of tobacco leaves has a certain influence on the conversion of free amino acids into Maillard products. Properly increase of the maturity of tobacco leaves contributes to the mutual conversion of free amino acids and the formation of Maillard reaction products.