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A ω-mercaptopropionic acid (MPA) self-assembled monolayer modified electrode (MPA/SAM/Au) on a gold electrode has been fabricated. The characterization of the MPA/SAM/Au was investigated using attenuated total reflection-fourier transform infrared (ATR-FTIR) and A.C. impedance. The electrochemical behaviors of p-aminophenol (p-AP) were studied at the MPA/SAM/Au by cyclic voltammetry and semi-derivative voltammetry (SDV) in BR buffer solution. The modified electrode shows excellent electrocatalytic activity for the redox of p-AP and accelerates the electron transfer rate. The diffusion coefficient (D) is 4.55 × 10−6 cm2 s−1. The oxidative peak current increases linearly with the concentration of p-AP in the range of 4.0 × 10−8 ∼ 8 × 10−6 mol L−1 and 1.0 × 10−5 ∼ 2 × 10−4 mol L−1 by square wave voltammetry response, respectively. The detection limit (three times the signal blank/slope) is up to 1.2 × 10−8 mol L−1. The modified electrode is able to eliminate the interference of p-benzenediol, o-benzenediol and o-AP at a 40-, 90- or 70-fold concentration of p-AP, and it has been satisfactorily used for the determination of the real sample.