Upconverting nanoparticles (UCNps) possess the ability to convert light from low to high energy. In particular, the absorption of radiation by these nanomaterials in the near-infrared region of the spectrum, and their subsequent emission in the visible region, is of great interest for biomedical applications. Conventional antitumor therapies often produce a high degree of side effects. Consequently, it is proposed to investigate the development of less invasive alternative therapies as photothermal therapy, using UCNps. The upconversion property could be achieved by incorporating dopants (rare earths and transition metals) in fluorine-based crystalline environments. On the other hand, it is important to control the size of the nanoparticles for their use in biomedical applications, for that reason we plan to obtain nanoparticles with an approximate size less than 50 nm. In the present work, the development of KMgF3 fluoroperovskite nanoparticles by solvothermal synthesis is presented, applying a factorial experimental design which consists of four factors (temperature, time and two limiting reagents) at two levels and choosing the average particle size as a variable response. The samples were characterized by powder X-ray diffraction and Transmission Electron Microscopy, in order to know the crystalline phase and particle size. As a result, KMgF3 nanoparticles with an average size between 13 and 31 nm were obtained. In addition, data obtained were statistically processed by Analysis of Variance, to determine the significant factors and their interactions, achieving the optimal synthesis conditions. From these results, a series of samples doped with Mn2+ and/or Nd3+ were obtained in order to find the optimal dopant concentrations for efficient upconversion properties. Our work is the starting point for the development of UCNps allowing them to be applied in future antitumor therapies.