The dead surface layer of blue-emitting ZnS:Ag,Al phosphor was investigated by means of cross-sectional transmission electron microscopy for the first time. It was found that the electron radiation-induced damage at 6 keV excitation gives rise to (i) the decomposition of ZnS and the evolution of sulfur at the topmost surface, and (ii) the nucleation and multiplication of lattice defects, for example vacancy clusters and dislocations, at the electron penetration layer of approximately 300 nm. It was estimated that the decompositon rate of ZnS and the nucleation rate of defects depend on the crystallinity and surface roughness of the ZnS host crystal. Based on these evidences, necessary specifications of ZnS:Ag,Al phosphor for the long lifetime of field emission display are to be discussed in this preliminary work.