In this paper, gold nanoparticles were obtained by gamma-ray irradiation in aqueous solution containing HAuCl4 as the precursor. The reaction generated particles that contained no foreign stabilizer. The additive-free gold nanoparticles were employed to modify glassy carbon electrode for the detection of dopamine in phosphate buffer solution. Electrochemical properties of the modified electrode were examined by cyclic voltammertry and electrochemical impedance spectroscopy. Linear analytical curves were obtained in the range from 2 to 120 μM for dopamine by differential pulse voltammetry, with the detection limit of 0.13 μM (signal-to-noise ratio of 3). The relatively simple synthetic method of the gold nanoparticles and excellent electrochemical performance with high reproducibility and stability of the detection were promising for further applications.