L: -Aspartyl L: -amino acid methyl ester was synthesized using a mutant of a thermostable leucine aminopeptidase from Streptomyces cinnamoneus, D198 K SSAP, obtained in previously. A peptide of high-intensity sweetener, L: -aspartyl-L: -phenylalanine methyl ester, was selected as a model for demonstrating the synthesis of L: -aspartyl L: -amino acid methyl ester. The hydrolytic activities of D198 K SSAP toward L: -aspartyl-L: -phenylalanine and its methyl ester were, respectively, 74-fold and fourfold higher than those of wild type. Similarly, the initial rate of the enzyme for L: -aspartyl-L: -phenylalanine methyl ester synthesis was over fivefold higher than that of wild-type SSAP in 90% methanol (v/v) in a one-pot reaction. Furthermore, other L: -aspartyl L: -amino acid methyl esters were synthesized efficiently using D198 K SSAP. Results show that the substitution of Asp198 of SSAP with Lys is effective for synthesizing L: -aspartyl L: -amino acid methyl ester.