ZnS-based phosphors were coated with ZnO and Y2O3 nanosize particles via a colloid chemical route. Degradation by electron bombardment was improved by modification with ZnO and Y2O3, respectively. In a usual measurement using a metal sample holder, ZnO showed better degradation suppression than Y2O3. In contrast, the difference vanished when an insulating holder was used. Thus, coating of ZnO with higher electrical conduction suppresses the degradation more efficiently, as compared with Y2O3.