A multiwall carbon nanotube modified electrode prepared by incorporating multiwall carbon nanotubes in the electrode of a sensor and naphthol green as a homogeneous mediator was used as a voltammetric sensor for the determination of N-actylcysteine (N-AC) in the presence of tryptophan (Trp). The voltammograms of differential pulse voltammetry of N-AC in a mixture with Trp were separated from each other by a potential difference of 200 mV, which allowed the determination of both N-AC and Trp simultaneously. Under the optimum conditions, the electrocatalytic currents increased linearly with N-AC concentration in the range of 0.25-400 μmol/L (two linear segments with different slopes). The detection limit for N-AC was 0.08 μmol/L. The kinetic parameters of the system were determined using electrochemical methods. The method was applied for the determination of N-AC in drug and urine samples.