We report on fabrication of Er3+- activated LaF3 nanocrystals in transparent glasses using an original technique, which combines both heat treatment, below glass crystallization temperature, and ultraviolet laser irradiation at 244 nm. The main advantage of this method is to control the spatial localization of the nanoparticles in the glass sample, whereas annealing solely at the crystallization temperature leads to a fully crystallized glass sample. Thermal differential analysis was used to determine the crystallization temperature of the sample. The photoluminescence spectra behaviour of Er3+ ions, collected from the UV-irradiated and unirradiated regions, allowed us to follow and to distinguish the structural changes in the glass network under heat treatment and ultraviolet exposure.