This work reports on the preparation of a luminescent blue-emitting rare earth (RE) Tm-doped oxide phosphor. Nanocrystalline RE2O3:Tm3+ particles were prepared via the combustion method using citric acid, glycine, or urea as fuels. Samples were doped with different percentages of the activator Tm3+. The post-annealing treatment was performed in air for all the samples, at temperatures ranging from 800 to 1100°C, for 4 h. The samples were characterized by X-ray diffraction (XRD), photoluminescence spectroscopy (PL) and scanning electron and transmission microscopies (SEM and TEM) in order to determine the best synthetic procedure. The post-annealed powders showed blue emission with maximum at 452 nm characteristics for Tm3+ transition 1D2 → 3H4 (under UV excitation at 360 nm). Samples, presented a tri-dimensional porous structure (50-200 nm) formed of spheroid particles with a diameter between 20 and 60 nm. The best luminescent material was obtained when urea was used to prepare nanoparticles of Gd2O3 doped with 0.5% Tm3+, and 1100°C was used as the post-annealing temperature.