In this work, we describe simultaneous determination of two amino acid, L-cysteine and tryptophan, using a carbon nanotubes paste electrode (CNPE) modified with p -aminophenol as a mediator in aqueous solution at a pH=6.0. Cyclic voltammetry (CV), differential pulse voltammetry (DPV), double potential step chronoamperometry and electrochemical impedance spectroscopy (EIS) were used to investigate the electrochemical behavior of L-cysteine at a chemically modified electrode. The results showed an efficient electrocatalytic activity of the electrode for the oxidation of L-cyseine, which leads to a reduction in its overpotential more than 0.55 V. Using differential pulse voltammetry, L-cysteine and tryptophan in a mixture can each be measured independently from each other with a potential difference of 0.60 V. The results showed that tryptophan cannot catalysis at a surface of the modified carbon nanotubes paste electrode. The peaks current was linearly depend on L-cysteine and tryptophen concentrations in the concentration range of 0.5–100.0 ?molL –1 L-cysteine and 10.0–300.0 ?mol L –1 tryptophan. The detection limits for L-cysteine and tryptophan were 0.3 ?mol L –1 and 5.7 ?mol L –1 , respectively. The diffusion coefficient and the rate of electro-oxidation of L-cysteine at the surface of p -aminophenol-modified carbon nanotubes paste electrode was determined 6.20×10 ?4 cm 2 s ?1 and 4.36×10 3 M -1 s -1 respectively for the experimental conditions.Cyclic voltammetry (CV), square wave voltammetry (SWV), double potential step chronoamperometry and electrochemical impedance spectroscopy (EIS) were used to investigate the electrochemical behavior of gluthathione (GSH) at a chemically modified electrode prepared by incorporating p -aminophenol ( p -AP) into multi-wall carbon nanotubes paste matrix.The resulthas been shown by these methods that p -AP can catalyze the oxidation of glutathione in aqueous buffer solution (pH=5.0) and produces a sharp oxidation peak current at about +0.21 vs. Ag/AgCl reference electrode. The square wave voltammetric peak current of GSH increased linearly with the corresponding GSH concentration in the two linear ranges of 0.2 – 4.3 ?mol L –1 and 4.3-100.0 ?M with a detection limit of 0.09 ?mol L –1 .The catalytic rate constant for oxidation of GSH at the p -APMWCNTPE was also determined and found to be about 9.30×10 3 M -1 s -1 . The diffusion coefficient of GSH was also estimated as 6.00×10 ?4 cm 2 s ?1 for the experimental conditions, using chronoamperometry.