Abstract In this study, a two-step pretreatment method was used to improve the sinterability of zirconium diboride (ZrB2). Raw powder was initially crushed using a planetary or SPEX mill to an average size of 0.5–0.6 μm. Oxide impurities on the surface of the ZrB2 powder were then removed with a dilute hydrofluoric solution. The relative density of the sintered ZrB2 increased as the concentration of residual oxides was reduced. When the concentration of oxide impurities was less than 0.5 wt.%, a significant increase in the relative density was observed. Through the proposed pretreatment process, we produced sintered ZrB2 ceramics with a relative density of 99.5% without the use of sintering additives. The results experimentally demonstrate the fact that oxide impurities on the surface of ZrB2 powder are one of the main reasons for inhibiting densification. The sintering behavior of ZrB2 was improved by using mechanical and chemical pretreatment methods.