High-purity zirconium diboride (ZrB2) powders with submicrometer particle size were synthesized by borothermal reduction of nanometric ZrO2 powders in vacuum. The reaction process was experimentally and thermodynamically assessed. B2O3 was identified as a possible intermediate reaction product. ZrO2 completely converted to ZrB2 when thermally treated at 1000 degrees C for 2 h in a vacuum, but the removal of residual boron-related species required a temperature above 1500 degrees C. ZrB2 powders obtained at 1000 degrees-1200 degrees C showed a faceted morphology, whereas those prepared above 1500 degrees C had a nearly spherical morphology. The particle size that was calculated from the measured surface area increased with the increasing synthesis temperature from 0.15 mu m at 1000 degrees C to 0.66 mu m at 1650 degrees C. The oxygen content of the ZrB2 powders synthesized at 1650 degrees C was as low as 0.43 wt%.