Y2O3:Eu3+ nanospheres with sizes of 40-334 nm in diameter were obtained using a low-cost co-precipitation method followed by a thermal annealing process. The sizes of the nanospheres were controlled by tuning the synthesis time and annealing temperature. X-ray diffraction patterns and scanning electron microscopy images were used to determine the structure, shape, and size of the obtained nanoparticles. The optical properties of the nanospheres were investigated with measurements of the photoluminescence excitation and emission spectra. A phase transformation of the nanospheres from an amorphous structure to a cubic crystalline Y2O3 structure was observed when the annealing temperature was higher than 500 °C. Intense red photoluminescence emission and UV excitation of the nanospheres with a crystal structure were identified. In addition, the optimal concentration of dopant (Eu3+) for the red emission was determined to be ~8 mol%. The unique structural and optical properties of the Y2O3:Eu3+ nanospheres could lead to efficient red LED-phosphors for use in generating white light with GaAlN-based UV LEDs.