LaF3:Eu3+ and GdF3:Eu3+ nanoparticles were prepared by a co-precipitation method in the presence of the chelating agent,citric acid.The structural properties of the products were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM).The average crystallite size was estimated from the full-width at half-maximum (FWHM) of the diffraction peaks by the Scherrer equation.The sizes of the nanoparficles were 12 um for LaF3:Eu3+ and 17 nm for GdF3:Eu3+.The luminescent properties of the nanoparticles were investigated by excitation and emission spectra.Energy transfer from Gd3+ to Eu3+ was observed.