Abstract: By using ¹⁵NH₄Cl as starting material, an improved Gabriel preparation method was used for the synthesis of S-benzyl -D,L -cysteine, which was further converted to S-benzyl-L-cysteine by optical resolution in the presence of aminoacylase. After the removal of protective group, the target compound ¹⁵N -L-cystine was obtained. The main controlling factors of the reactions were investigated. Under the optimized conditions, the total yield of L -cystine reached 13.5% based on ¹⁵NH₄Cl consumed. The product was characterized by HPLC， IR，MS -EI and elemental analysis. The dilution of enrichment was not found in the process.