The nanocrystals (NCs) of tetragonal barium yttrium fluoride (BaYF5) doped 1 mol% Ln(3+) (Ln = Er, Tm, Ho) and 20 mol% Yb3+ with different morphologies and sizes have been successfully synthesized through a facile hydrothermal method. The influences of pH values of the initial solution and fluorine sources on the final structure and morphology of the products have been well investigated. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM) were used to characterize the size, structure and morphology of these samples prepared at different conditions. And it is found that BaYF5:Yb/Ln NCs prepared at pH value of 10 using NaBF4 as F- source have a uniform spherical morphology with average diameter of 25 nm. Additionally, the up-conversion (UC) properties of Yb/Er, Yb/Tm, and Yb/Ho doped BaYF5 nanoparticles were also discussed. Under 980 nm laser excitation, the BaYF5:Yb/Er, BaYF5:Yb/Tm, and BaYF5:Yb/Ho NCs exhibit green, whitish blue, and yellow green UC luminescence, respectively. The luminescence mechanisms for the doped lanthanide ions were thoroughly analyzed. (C) 2011 Elsevier Inc. All rights reserved.