Abstract: In order to find out the differences of chemical components between mildewed and normal tobacco leaves, the contents of 17 free amino acids in cured tobacco leaves were determined by an amino acid analyzer. The results showed that: 1)The total amount of free amino acids in normal leaves was 3.6 times of that in mildewed leaves. 2)Proline was the major amino acid and accounted for 75.58% of total amino acids in normal leaves. Whereas the major amino acids in mildewed leaves were aspartic acid, histidine, glutamic acid and phenylalanine, which accounted for 68.69% of total amino acids. 3) The contents of proline, aspartic acid, histidine, glutamic acid and phenylalanine differed more obviously between mildewed and normal leaves. Therefore, the decrease of total amount of free amino acids and the variation of some free amino acids can be used as the biomarkers in mildewed tobacco leaf identification.