ZnS:Ag,Cl phosphor particles were coated discretely by ZnO nanoparticles to prevent degradation by electron bombardment at 7kV. The degradation of ZnS:Ag,Cl modified with ZnO nanosize particles was reduced by 40% after 9h exposure to 7kV electron beams, in comparison with that without ZnO nanosize particles. The predominant mechanism of degradation prevention is to suppress surface oxidation of ZnS by residual oxygen during cathodoluminescence (CL) experiment. Increased electric conduction and associated protection from local heating are the main reasons for degradation suppression, as confirmed by experimental measurement of degradation for the insulating ZnS:Ag,Cl phosphor.