Tm3+, Yb3+-codoped SrF2 nanoparticles were synthesized through a facile hydrothermal technique. Citrate ions were introduced as the capping agent into the reaction. Upconversion nanoparticles were characterized by field emission scanning electron microscopy (FESEM), Energy dispersive x-ray spectroscopy (EDS), x-ray diffraction (XRD), Dynamic light scattering (DLS), Zeta potential, Fourier transform Infrared spectroscopy (Ft-IR), and the 980 nm laser induced photoluminescence spectroscopy. Rare-earth ions (Na+), which are the cations of citrate salts, are incorporated into the structure to act as charge compensators. Upconversion emission in the visible and NIR region was observed by the 980 nm irradiation. Nanoparticles with a narrow size distribution and a uniform morphology were directly dispersible in water, forming a quite transparent suspension. Nanoparticles size was approximately 10 nm. High penetration of the Near-Infrared light into the body tissue makes these nanoparticles appropriate for tumor targeting in the deeper tissues for the purpose of bioimaging and photodynamic therapy.