In this study, sub-10 nm BaLuF5 nanocrystals with cubic phase structure were synthesized by a solvothermal method using oleic acid as the stabilizing agent. The as-prepared BaLuF5 nanocrystals were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), and analyzed by the upconversion (UC) spectra. The TEM results reveal that these samples present high uniformity. Compared with Gd-free samples, the size of BaLuF5:Yb/Er doped with 10% Gd3+ decreased to 5.6 nm. In addition, BaLuF5:Yb/Tm/Gd upconversion nanoparticles (UCNPs) presented efficient near-infrared (NIR)-NIR UC luminescence. Therefore, it is expected that these ultra-small BaLuF5 nanocrystals with well-controlled shape, size, and UC emission have potential applications in biomedical imaging fields. (C) 2014 Elsevier Ltd. All rights reserved.