Catalyst of in-situ Cu nanoparticles (CuNPs) is electrodeposited on boron-dopeddiamond electrode for a thiol compound of 2-mercapto ethanesulfonic acid (MCES)determination. The morphology of CuNPs was investigated by scanning electron microscopy(SEM). The SEM micrograph of CuNPs was shown spherical in shape and about 100 nm indiameter. Cyclic voltammetric, and square wave voltammetric results showed that the CuNPsacts as an efficient material for the electrocatalytic oxidation of MCES. According to thevoltammetric studies, the MCES electrooxidation on the presented in-situ CuNPs electrodewas occurred at lower potential than the absented CuNPs electrode and also thevoltammogram was shown a well-defined oxidation peak. The in-situ CuNPs modifiedboron-doped diamond electrode was used as a sensor for determination of MCES with agood dynamic range of 100 to 1000 μM with a high sensitivity of 2.59 nA/μM and a lowdetection limit of 30 μM (S/N=3).