Potential-dependent surface-enhanced Raman scattering spectra reveal that the orientation of o-phenylenediamine (OPD) molecules adsorbed on a Au electrode depends on the electrode potential and electropolymerization of OPD can be carried out at + 0.3 V. The electrochemical-assembly technique was successfully applied to the fabrication of a nano-structured o-phenylenediamine oligomer (POPD) film on a bare Au substrate and on a p-aminothiophenol (PATP) self-assembled Au substrate in phosphate buffer solution (pH = 7.0). Electrochemistry and scanning tunneling microscopy measurements indicate that a POPD/Au(111) surface displays a nano-scale dot array structure. At the same time, a POPD/PATP/Au(111) surface shows a nano-scale line array structure.