Yttrium-aluminum garnet (YAG):Ce3+ nanophosphor powder is prepared from yttrium acetate, cerium acetate, and aluminum isopropoxide by a one-step glycothermal method, i.e., by autoclave treatment in 1,4-butanediol at 300 degrees C for 2 h. The gradual decrease in the intensity of photoluminescence corresponding to the 5d -> 4f transition of Ce3+ is observed under the continuous irradiation of blue excitation light. This photobleaching behavior is not observed for the micrometer-sized bulk. Two-step glycothermal synthesis, i.e., the repeated autoclave treatment, causes the growth of nanoparticles and the improvement of crystallinity, and hence suppresses the photobleaching. The suppression of photobleaching is more prominent for the sealed transparent film produced from the paste compared with the powder samples. The photobleaching is attributed to the photo-oxidation of Ce3+ to Ce4+ near the surface of nanoparticles with a high proportion of surface atoms, as confirmed by electron energy loss spectroscopy.