Crown-ether-modified silica nanoparticles (Crown SNPs) were successfully prepared by simple chemical modifications of silica nanoparticles. The organic moiety introduced into silica nanoparticles was characterized by solid-state C-13 NMR and elemental analysis. No morphological damage of silica nanoparticles was recognized according to SEM observation. When an alkali metal salt was added to a solution containing Crown-SNPs, the absorbance changes, whose extent was dependent on the kind and the concentration of a metal ion, were observed.