The present work reports on the preparation of boron carbide nanoparticles by the reduction of boron oxide with magnesium in the presence of carbon using the mechanochemical processing. The phase transformation and microstructure of powders during ball milling were investigated by X-ray diffractometry (XRD) and scanning electron microscopy (SEM). The results showed that during ball milling the B2O3-Mg-C reacted with a self-propagating combustion mode producing MgO and B4C compounds. To separate B4C from the milled powder mixture, an appropriate leaching process was used. After leaching, the purified powder mixture was characterized using XRD and transmission electron microscope (TEM). XRD studies indicated that the prepared particles were single phase crystalline B4C. Moreover, TEM studies showed the size of B4C particles were ranging from 10 to 80 nm. (c) 2010 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.