Simultaneous determination of N-acetyl-L-cysteine (NAC) and Tryptophan (Trp) has been studied at the surface of glassy carbon electrode (GCE) modified with multi-walled carbon nanotubes (MWCNTs) in the presence of 4-chlorocatechol as homogenous electrochemical catalyst in aqueous media. The electrocatalytic properties of GCE modified with MWCNTs in the presence of 4-chlorocatechol toward the electrocatalytic oxidation of NAC and Trp was studied using cyclic voltammetry (CV), double-step potential chronoamperometry and differential pulse voltammetry (DPV). The results has shown that NAC participates in Michael type addition reaction with electrogenerated quinone from electrooxidation of 4-chlorocatechol at MWCNT/GCE to form the corresponding thioquinone derivative. The reoxidation of the adduct leads to increase in the oxidative current which is proportional to the concentration of NAC. Differential pulse voltammogram peak currents of NAC and Trp increased linearly with their concentration at the ranges of 5-60 μM and 5-50 μM, respectively and the detection limits for NAC and Trp were sequentially 3.427 μM and 2.494 μM. The proposed method was successfully used for the determination of NAC in pharmaceutical samples and the obtained results were found to be reasonable.