The reactivity of starting materials and heat treatment conditions for the preparation of ZrB₂ fine powders using a thermite method were examined in the ZrO₂-B₂O₃-Mg and ZrO₂-B-Mg systems. The removal treatment of by-products was also investigated. In the ZrO₂-B₂O₃-Mg and ZrO₂-amorphous B-Mg systems, the reduction of ZrO₂ and B₂O₃ by Mg and the boriding reaction of α-Zr and Mg occurred concurrently. The ZrB₂ fine powders of single phase were successfully prepared at 1100°C in the ZrO₂-B₂O₃-9Mg (molar ratio) system and at temperatures above 700°C in the ZrO₂-2B-5Mg (molar ratio) system. Washing of the products with deoxidized acidic solutions in nitrogen atmosphere was effective to separate ZrB₂ from MgO and MgB₂, and to inhibit the oxidation of the ZrB₂, too. Mg content in the powders after the acid treatment was 0.6-1.0wt%.