In this paper, the NaYF 4:Yb, Tm upconversion (UC) nanoparticles (NPs) were synthesized using a solvothermal approach, and the core-shell structured NaYF 4:Yb, Tm@SiO 2 NPs, coated with a thin layer of SiO 2 on the surface of the NaYF 4:Yb, Tm NPs, were prepared by a typical Stöber method. X-ray diffraction (XRD), transmission electron microscopy (TEM), and luminescence spectroscopy were applied to characterize these samples. The obtained core-shell structured NaYF 4:Yb, Tm@SiO 2 NPs exhibited a perfect spherical morphology with narrow size distribution and smooth surfaces. Under 980 nm excitation, NaYF 4:Yb, Tm and NaYF 4:Yb, Tm@SiO 2 samples showed intense ultraviolet UC luminescence, which originated from the 1D 2 → 3H 6, 1I 6 → 3F 4 transitions of Tm 3+. These NPs have great potential for applications as fluorescent labels, imaging probes, optical storage, photodynamic therapy (PDT) in deep tissue, and solid-state lasers.