The mechanism of the carbothermal method for synthesizing titanium diboride (TiB2) powder has been studied. Mixtures of TiO2, H3BO3 and carbon were heated in an argon atmosphere at 1000-1600 degrees C. The effect of the molar ratio and holding time on the phase evolution was studied by X-ray diffraction. The products were also characterized by scanning electron microscopy observations and particle size measurements. For a composition with a molar ratio of TiO2:H3BO3:C = 1:2.4:5 heated for 1 h, the simultaneous presence of TiC and TiB2 phases at 1100 degrees C and the transformation of TiO2 to Ti2O3 at 1200 degrees C and higher confirms that TiB2 synthesis is based on a TiC formation mechanism, in which TiC may be formed from a reaction between TiO2 or Ti2O3 and carbon. Then TiC may react with liquid B2O3 and/or gaseous B2O2 to form the TiB2 phase. The reaction is completed at 1500 degrees C. Also by increasing the molar ratio of boric acid to 3, the impurities decreased considerably and pressing of the material had an obvious effect on decreasing the impurities, due to an increase of the surface contact of particles, which causes an effective inhibition of boron escape from the reaction chamber. Under these experimental conditions, a relatively narrow size distribution of TiB2 particles was produced. When the reaction time increased to 1.5-2 h, grain growth of particles occurred. Therefore, a wider distribution of particle size was obtained. (C) 2011 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.