The effect of a surface plasmon (SP) on the yellow emission of Ce doped Y3Al5O12 (YAG:Ce) phosphor thin films was studied. The coating of YAG:Ce thin films with silver nanoislands on a 3-nm-thick SiO2 spacer significantly enhanced their photoluminescence (PL). The PL enhancement is caused by the resonant coupling of photoemission in YAG:Ce into SP and the electric field of incident light at the metal interfaces. Varying the mass thickness and the annealing condition of the Ag layer indicated that the PL intensity was highest at an Ag mass thickness of 20 nm. Therefore, yellow emission was enhanced by tuning the matching conditions of thin film phosphor-SP coupling by controlling the structure of silver nanoislands. The PL intensity can be further and remarkably enhanced by capping Ag islands with a 10-nm-thick SiO2 layer as a dielectric medium.