In this study, an electrochemical sensor was used for simultaneously determination of N-acetyl cysteine (NAC) and acetaminophen (AC) using differential pulse voltammetry. Modification on the surface of a glassy carbon electrode (GCE) was carried out using multi-walled carbon nanotubes (MWCNTs) and Nafion (NF). The morphology and structure of the MWCNT-NF nanocomposites were characterised by scanning electron microscopy (SEM). The modified sensor showed excellent performance for the oxidation reactions of NAC and AC. Using the modified sensor, the oxidation peak potentials of NAC and AC were separated by a potential difference of 350 mV, while they were coincident at the bare electrode. Moreover, the current densities were significantly improved using the modified sensor. The modified sensor displayed a linear response range of 20-500 mu M NAC and the detection limit was 0.43 mu M (S/N = 3). Under optimal conditions, the modified sensor exhibited excellent stability and selectivity, with high precision and accuracy. With the satisfactory results obtained from the real samples analysis, the modified sensor can be used for routine analysis. To the best of our knowledge, this is the first study in the literature that detected NAC in the presence of AC at GCE/MWCNT-NF modified sensor.