The samples of LaF3∶Sm3+, LaF3∶Eu3+ and LaF3∶Sm3+/Eu3+nanocrystals with high quality mono-disperse and uniform sizes were synthesized with hydrothermal method. The crystallographic phase, surface morphology, crystalline sizes and fluorescence properties of Sm3+/Eu3+ sole- and co-doped nanocrystals were characterized with X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and photoluminescence (PL) spectroscopic technique, respectively. The results of XRD and TEM show that the microstructure of the nanocrystals is hexagonal, with the average size about 40 nm. Using 442 nm He-Cd continuous wave (CW) laser to pump the Sm3+ ions doped in the LaF3∶Sm3+/Eu3+nanocrystals, the typical fluorescence emissions originating from the Eu3+ions were observed in the emission spectra, and that the energy transfer from Sm3+ions to Eu3+ions was completed. The mechanism and efficiency of the energy transfer from Sm3+ions to Eu3+ ions were investigated and discussed systematically based on the spectroscopic method. It is shown that the energy transfer of Sm3+→Eu3+ is attributed to the cross-relaxation between the excited state 4G5/2 of Sm3+ ion and the 5D1 and 5D0 states of Eu3+ ion. Meanwhile, the intensities of the characteristic fluorescence emissions of Eu3+ ions become stronger and stronger with the increase of the Eu3+ doping concentration, which suggest that the efficiency of energy transfer from Sm3+ ions to Eu3+ ions can be effectively improved by increasing the doping concentration of Eu3+ acceptor.