Zirconium diboride (ZrB2) is a typical ultra-high temperature ceramic material (UHTC). In this work, the preparation of ZrB2 by reducing zirconia (ZrO2) with calcium hexaboride (CaB6) was studied in detail. With the assistance of molten salt, the reduction rate was dramatically enhanced. Additionally, the results showed that ZrB2 products with different particle shapes were prepared by changing the type of molten salt (NaCl, KCl, MgCl2, and CaCl2). However, the particle size of products could not be properly refined when nano-ZrO2 was used as the raw material. During the process of reducing ZrO2 by CaB6, B2O3 could be formed, which would cause the loss of B source. To decrease the consumption of CaB6, Ca-assisted reduction was performed, and monophase ZrB2 was obtained. The varied morphologies of ZrB2 particles obtained by Ca-assisted reduction were the result of complex reaction mechanisms.
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