Monodisperse red phosphor particles 100 nm in diameter with the Lu1.90Eu0.10O3 composition have been prepared using the developed technique for synthesizing spherical colloidal lutetium oxide particles with a size dispersion in the range 10-15%. The structure of spherical nanoparticles has been investigated, their excitation and photoluminescence spectra have been analyzed, and the lifetime of the (5) D (0) excited state of Eu3+ ions has been considered. It has been found that the luminescence decay time for spherical particles increases by a factor of 1.39 compared to that for a powdered phosphor Lu2O3: Eu (5 at %) prepared and treated under the same temperature conditions as the Lu2O3: Eu (5 at %) spherical particles. This effect has been associated with the change in the photonic local density of states in spherical optical cavities consisting of particles of the phosphor.