Abstract: OBJECTIVE To synthesize l-menthyl diclofenacate (Compound 3 ), an impurity in diclofenac sodium gel, as a reference substance for the quality control of the gel. METHODS Diclofenac acid (Compound 2 ) was prepared by acidification of diclofenac sodium with dilute hydrochloric acid (Compound 1 ). l-Menthyl diclofenacate ( 3 ) was synthesized by straightforward esterification of Compound 2 with l-menthol using EDCI as the dehydrator and DMAP as the catalyst. RESULTS & CONCLUSION The structure of the title Compound 3 was confirmed by IR, ¹H-NMR and HR-MS(ESI). The purity was about 99.60% as judged by HPLC.