The present invention aims at realizing a PDP and a mercury-free
fluorescent lamp feasible to maintain excellent luminescent
characteristics over long periods by suppressing time-lapse changes in
luminescent characteristics of a phosphor that is excited by vacuum
ultraviolet light to thereby emit light. To accomplish this object, the
oxide phosphor of the invention comprises individual particles, each of
which has a region at and near the surface thereof modified, and the
elemental composition of the surface region is in a more oxidized state
than that of the internal region of the particles. Alternatively, the
surface region has more halogen or chalcogen in the elemental
composition. In the phosphor treatment method of the invention, the
surface region of individual phosphor particles is selectively modified
by (i) forming a highly reactive gas atmosphere by exciting gas which
contains reactive gas, and (ii) exposing the phosphor to the gas
atmosphere.