Calcium hexaboride (CaB6) powder was synthesized by carbothermal reduction via the transient boron carbide (B4C) formation starting from a condensed boric acid (H3BO3)-poly(vinyl alcohol) (PVA) product. The effect of the size of calcium carbonate (CaCO3) particles on the formation behavior of CaB6 and the obtained particle morphology was investigated in this study. CaB6 powder was prepared using CaCO3 powders with microsize or nanosize particles. The CaB6 formation reaction was accelerated at lower temperature and shorter heat treatment time when using nanosize CaCO3 particles. The complete formation of CaB6 was achieved at 1400 degrees C for 3 h in an Ar flow. Furthermore, CaB6 powder with nanosize particles was obtained. The precursor powder obtained using nanosize CaCO3 particles transiently formed fine B4C and calcium borate particles, which are reactive species of CaB6, leading to the facile formation of fine CaB6 particles. (C) 2017 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.