64keV Ni ion implantation was performed at room temperature up to a dose of 131017 cm22 in a-Al2O3 single crystals. The charge states, structure, and optical properties of metallic embedded Ni nanoparticles were studied by using x-ray photoelectron spectroscopy (XPS), transmission electron microscopy, and optical spectroscopy, respectively. XPS analysis showed that implanted Ni ions are mainly in charge state of metallic Ni0. Nanoparticles distributed from the surface to 30 nm below the surface were observed in a high-angle annular dark-field image. The size of nanoparticles ranges from 1 to 5nm in diameter. A high-resolution electron microscopy image indicated the Ni-implanted area had been entirely amorphized. A new broad absorption band centered at 400nm appeared in the optical absorption spectrum of the as-implanted crystal, due to surface plasma resonance of Ni nanoparticles. We did not find any emission band in the as-implanted sample under a Xe lamp excitation wavelength of 250-430nm in a spectrophotometer.