Poly 4-aminobenzene sulfonic acid modified glassy carbon electrode (poly(4ABSA/GCE)) was prepared with electrochemical polymerization technique in phosphate buffer solution (PBS) (pH 7.0). The modified glassy carbon electrode (GCE), which has high electrooxidation ability in the pH range of 5-10 with cyclic voltammetry (CV) and differential pulse voltammetry (DPV) techniques, was successfully developed for the electrochemical determination of quercetin (QR). However, the QR or the products of the oxidation reaction are adsorbed strongly on the modified glassy carbon surface. Therefore, when subsequent sweeps of the CV and DPV voltammograms were recorded, a decrease was observed at the peak current and at peak potential there was a shift to a lower positive potential. The best results for the quantitative determination of QR were obtained by DPV technique in 0.1 M PBS (pH 8.0). A linear calibration curve for DPV analysis was constructed in the QR concentration range of 7x10(-5) to 9x10(-4) M. The method was applied for the analysis of QR in onion peel.