The powder samples of Sm3+/Eu3+co-doped in LaOF nanocrystals were synthesized by using a hydrothermal-sintering technique. The characterization results of XRD and TEM suggests that the structure of the nanocrystals is tetragonal, and its size is between 60～80 nm. Using 442 nm laser to pump the LaOF∶Sm3+/Eu3+ nanocrystals samples, the energy transfer effect from Sm3+ ions to Eu3+ ions was completed, and the fluorescence emissions originating from the 5D0 level of Eu3+ were observed. The luminescence properties of the LaOF∶Sm3+/Eu3+ nanocrystals system were analyzed with a spectroscopic method, and the mechanism and dynamic process of the energy transfer were explored. It was found that the efficiency of the energy transfer increase with the increase in the concentration of Eu3+ ions and the size of nanocrystals.