Magneto-optical Kerr effects, i.e., Kerr rotation and ellipticity, of nickel nanoparticles in silica glasses NOD fabricated by implantation of Ni negative ions of 60 keV were studied in the wavelength region of 200-900 nm under a magnetic field up to similar to 21 kOe at room temperature. Experimental results of the Kerr spectra of the Ni nanoparticles were compared with calculations using the improved Lissberger-Saunders theory which is an extension of Maxwell-Gamett theory to magnetic composites including the size-dependent Drude free-electron function and the size-independent bound-electron term. Although the calculated spectra qualitatively agree with the experimental results, quantitative agreements were not good. Possible origins of the discrepancy are discussed. (C) 2004 Elsevier B.V. All rights reserved.