ZrB2-based ultra-high temperature ceramics (UHTCs) were prepared from a mixture powder of Zr/B4C/Si with different ratio via reactive hot pressing. The experimental results showed that the sintering temperature above 1800°C was necessary for enhancing the activity of the powders and thus improving the densification of the product. The sinterability and densification properties of ZrB2-based UHTCs meliorated with the amount of Si increasing. However, many large ZrB2 agglomerates formed when the amount of synthesized SiC in the product reached 25vol%, which led to decrease the mechanical property. The composite had highest mechanical properties when the volume ratio of ZrB2: SiC: ZrC was 73.86:20:6.14, and its flexual strength and the fracture toughness were 645.8MPa and 5.66MPa·m1/2 respectively. The microstructure investigation showed the in-situ formed SiC and ZrC were located in the triple point of ZrB2 grains with a size less than 3μm.