The initial rough, faceted surfaces of boron-doped conductive polycrystalline diamond electrodes were smoothed by Ar+ ion sputtering at very low energy (50 eV). A lower background current was measured at the modified electrodes with a mirror-like appearance than at the initial polycrystalline electrodes. The electrochemical responses to several redox systems also showed a morphological-dependence in some cases. Here, we discuss the electrochemical properties of the diamond electrodes with the smoothed surfaces from the point of view of a novel electrode material.