The present report provides a new insight into the high purity synthesis of rare-earth ytterbium hexa-boride, YbB6 nanocrystals, without using a solid state reaction or metathesis reaction methods. Single phase YbB6 nanocrystals were synthesized for the first time by direct and dry milling of Yb2O3, B2O3 and Mg starting powders. The effects of milling period on the formation and crystallite size of milled nanoparticles have been investigated. Single phase Rietveld refinement confirmed the cubic structure in Pm-3m space group with the lattice parameters a = b = c = 3.1444(2) angstrom, and 68 nm particle size of YbB6 phase. The YbB6 nanocrystals revealed a weak ferromagnetism and plus paramagnetic behavior. The ferromagnetic and paramagnetic behavior of the YbB6 are correlated to the large surface-to-volume ratio of ball-milled nanocrystals, and the presence of boron-rich and boron-deficient phases. (C) 2019 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.