Monodisperse, well-defined, and photoluminescent CeF3, CeF3:Tb3+, and CeF3:Tb3+@LaF3 core/shell nanocrystals (NCs) with small particle sizes (<15 nm) have been successfully synthesized by a hightemperature thermolysis in high boiling solvent of oleic acid (OA) and 1-octadecene (ODE). The morphology, phase, composition, and the fluorescent properties of the as-synthesized samples were well characterized by means of transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), X-ray photoelectron spectra (XPS), photoluminescence (PL) spectra, and kinetic decays, respectively. It is found that the as-synthesized NCs consist of monodisperse nanoparticles with narrow size distribution, which can easily disperse in nonpolar cyclohexane solvent to form transparent colloid solutions. The CeF3, CeF3:Tb3+ and CeF3: Tb3+@LaF3 colloidal solutions show the characteristic emissions of Ce3+ 5d–4f (353 nm) and Tb3+ 5D4 / 7 FJ (J ¼ 63) transitions, respectively. The growth of the LaF3 shell on the CeF3:Tb3+ core NCs not only results in an increase of the average size, but significantly enhances the emission intensity and lifetime of the CeF3:Tb3+@LaF3 core/shell NCs with respect to those of CeF3:Tb3+ core NCs. Moreover, a transparent and fluorescent NCs/polydimethylsiloxane (PDMS) composite with regular dimension was also prepared by an in situ polymerization route.