A modified carbon paste electrode (CPE) was prepared based on a newly synthesized compound 14-(4-hydroxyphenyl)-14-H-dibenzo[a,j]-xanthene (HDX) and multi-wall carbon nanotubes (MWCNTs). At first, the redox properties of the modified electrode were studied by cyclic voltammetry (CV). Then, the modified electrode was used as an electrochemical sensor for the oxidation of cysteine (Cys). Under the optimum pH of 8.5, the overpotential of Cys oxidation decreased about 280 mV at the modified electrode rather than at the unmodified CPE. The differential pulse voltammetry (DPV) curve of Cys at the electrochemical sensor exhibited two linear dynamic ranges with a detection limit (3s_b) of 1.0 μM. Also, DPV was used for the simultaneous determination of Cys and tryptophan (Try) using the electrochemical sensor. The proposed electrochemical sensor was used for the determination of these substances in real samples (i.e. tablets and human serums).