Ultra-fine zirconium diboride (ZrB2) powders have been synthesized using inorganic-organic hybrid precursors of zirconium oxychloride (ZrOCl2.8H(2)O), boric acid, and phenolic resin as sources of zirconia, boron oxide, and carbon, respectively. The reactions were substantially completed at a relatively low temperature (similar to 1500 degrees C). The synthesized powders had a smaller average crystallite size (< 200 nm), a larger specific surface area (similar to 32 m(2)/g), and a lower oxygen content (< 1.0 wt%), which were superior to some commercially available ZrB2 powders. The thermodynamic change in the ZrO2-B2O3-C system was mainly studied by thermogravimetric and differential thermal analysis. The crystallite size and morphology of the synthesized powders were characterized by transmission electron microscopy and scanning electron microscopy.