The polyborosilazane (PBSZ) was employed as the sintering additive and the dense ZrB₂ ultra-high temperature ceramic samples were fabricated by hot pressing at 1900°C. The relative density of sintered ceramics is 96.4%. The PBSZ was converted to amorphous SiBCN ceramics after sintering. Mass transfer process was significantly improved in the amorphous region among the ZrB₂ grains. Densification temperature was reduced. The mechanical properties and high temperature oxidation resistance were also enhanced. The bending strength and fracture toughness of the ZrB₂ ceramics were 241 ± 10 MPa and 4.47 ± 0.5 MPa m^1/2, respectively. The crack was deflected along the grain boundaries. During high-temperature oxidation process, the grain boundary at SiBCN ceramic was oxidized into borosilicate glass and the low viscous glass was extruded out and spread over the surface quickly. The continuous oxide coating formed on the ZrB₂ ceramics surface prevented further oxidation.