We focus on the inorganic nanophosphor of YAG:Ce3+ nanoparticles as an alternative fluorescent probe instead of organic dyes for biological application. YAG:Ce3+ nanoparticles have the green emission at 530 nm under the excitation of blue light at 450 nm. Conventional biochemical equipments for organic dyes can be used for YAG:Ce3+ nanoparticles. SH groups were introduced by surface modification of YAG:Ce3+ nanoparticles with 3-mercaptopropyl trimethoxy silane, and were characterized by X-ray fluorescence analysis, FT-IR, and Ellman method. We demonstrated tagging avidin-immobilized agarose gel beads with biotinylated-YAG:Ce3+ nanoparticles, and tagging rabbit IgG-immobilized agarose gel beads with antirabbit IgG-immobilized YAG:Ce3+ nanoparticles.