Abstract: NaYF4:Yb, Er upconversion nanoparticles with strong fluorescent intensity were synthesized in the liquid paraffin-oleic acid system via a thermolysis method, with the rare earth stearate used as precursor. The as-prepared nanoparticles were characterized by TEM, XRD, FT-IR and luminescence spectroscopy. It was inferred that the mechanism of the synthesis is of a solid-liquid two-phase reaction. The effects of reaction time, temperature and solvent composition on the fluorescent performance of the nanoparticles were discussed in detail. As a result, the oleic acid-coated hydrophobic nanoparticles became highly hydrophilic after their surfaces were modified with carboxylic groups by use of the Lemieux-von Rudloff reagent.