Correlations between crystallite/particle size and the luminescent characteristics of submicrometer phosphors were investigated. Spray pyrolyzed europium doped yttrium oxide (Y2O3:Eu3+) particles were selected as a model material. Crystallite size and the particle size were controlled independently. The morphology and crystallite structure were characterized by field-emission scanning electron microscopy, high-resolution transmission electron microscopy, X-ray diffraction, and selected area electron diffraction. Photoluminescence (PL) properties were examined by spectrofluorophotometry and an absolute PL quantum efficiency (QE) measurement system. Chemical analyses and elemental mapping were conducted by Fourier transform infrared spectrophotometry and STEM equipped with energy dispersive spectroscopy, respectively. The results revealed that the PL properties were strongly dependent on crystallite size, particle size, surface chemistry, and the distribution of europium inside the phosphor particles. The PL intensities and QE increased with increasing crystallite size and particle size. The effect of crystallite size on PL properties played a more important role than that of particle size.