The ZrB2 powders with submicrometric particle size and low oxygen content were synthesized by a new borothermal reduction route using ZrO2 and excess boron as raw materials. The conventional process only contained the borothermal reduction of ZrO2 with boron at 1550 degrees C. By exploring the mechanism of ZrB2 particle coarsening during conventional process, a new borothermal reduction route was proposed. This route included the borothermal reduction of ZrO2 with boron at 1000 degrees C, washing by hot water, and the removal of residual boron oxides at 1550 degrees C, namely, two-step reduction plus intermediate water-washing (RWR). Using conventional process, the particle size and oxygen content of ZrB2 powder were about 2-3 mu m and 0.68 wt%, respectively. Based on the new route, the particle size and oxygen content of ZrB2 powder were about 0.4-0.7 mu m and 0.40 wt%, respectively.