Small-angle x-ray scattering (SAXS) technique has been used to investigate nano-sized powders of Y2O3, Y1.8Er0.2O3, and Y1.8Nd0.2O3, obtained by means of combustion synthesis. The results show that the powders have a mass-fractal behavior, with fractal dimension, Df, in the 1.7–1.8 range, and average particle sizes in the 20–30-nm range. These results are supported by transmission electron microscope observations. Moreover, from SAXS data, it has been possible to establish a narrow particle size polydispersity, as well as the presence of particle-diffuse or “fuzzy” interfaces. These results show, for the first time, the fractal behavior of materials prepared by combustion synthesis and are in agreement with a recently developed model. The nanostructural features are discussed within the framework of the peculiar optical properties of the doped luminescent materials.