Ultra-fine and high-pure zirconium diboride powders were prepared by carbothermal reduction boronization of zirconia/boron carbide/carbon mixtures. Fine-scale mixing of the reactants was achieved by solution-based processing in which zirconium oxychloride (ZrOCl2⋅8H2O) as the zirconia-bearing precursor was precipitated in the suspension of boron carbide (B4C) and carbon powders in water. The carbothermal reduction boronization reaction was substantially completed at relatively low temperatures (<1600°C) and the resulting products (ZrB2 powders) had small average grain sizes (1~2 μm) and high purity (>99.6 wt%). The experiments indicated that excessive B4C and C were necessary during the carbothermal reduction boronization because of volatilization of boron and carbon elements. The effects of temperature and holding time on the synthesis of ZrB2 powders were also discussed.