Citrate-stabilized gold (Au) nanoparticles of a diameter of 11 nm were immobilized on a monolayer of aminoundecanethiol (1-mercapto-11-aminoundecane) coated on a polycrystalline Au electrode surface. A quartz crystal microbalance measurement of the deposition process of the Au particles revealed that the saturated deposition amount is 10% of a 2D close-packed monoparticle layer, and this coverage was confirmed by an atomic force microscopy (AFM) observation. The An particle layer was characterized by the use of potential-modulated UV-visible reflectance spectroscopy (electroreflectance spectroscopy). The electroreflectance (ER) band at the plasmon absorption wavelength of the Au particles was positive-going, indicating that the plasmon absorption becomes stronger when changing the electrode potential to more negative. The plasmon absorption band shifted to longer wavelength when the electrode potential approaches to the potential of zero-charge, E-pzc, of the Au electrode. The ER signal intensity also showed a maximum around E-pzc. In light of good electronic communication between the Au electrode substrate and the deposited Au particles as demonstrated by reversible redox waves of solution phase species, the ER signal was interpreted as being originated from the potential-dependent charging-discharging process of the immobilized Au particles; a more negatively charged Au particle exhibits a stronger and sharper plasmon absorption band at a shorter wavelength. The implications of the ER signal were discussed.