The crystal structure, synthesis, and densification of zirconium diboride (ZrB2) are summarized in detail. In this chapter, ZrB2-ZrC-SiC ceramic was synthesized by reactive hot pressing a mixture of Zr, B4C, and Si powders. The thermal shock resistance of the ZrB2-SiC-ZrC ceramic was estimated by the water-quenching method and was significantly greater than that of a ZrB2-15vol.% SiC ceramic. The isothermal oxidation of the ZrB2-SiC-ZrC ceramic was carried out in static air at constant temperatures of 1000±15, 1200±15, and 1400±15 ºC for different amounts of time at each temperature. The mechanism of strength increase for the oxidized specimen indicated that the strength increased with the reaction rate, which was related to the rate of change in volume induced by reaction, initial crack geometry, elastic modulus, and surface free energy. The formation of oxide layers resulted in (I) repair of surface flaws, (II) increase in flexural strength, (III) appearance of a compressive stress zone beneath the surface oxide layers, (IV) decrease in thermal stress, and (V) consumption of thermal stress. These five aspects were favorable to the improvement of the thermal shock resistance of the ZrB2-SiC-ZrC ceramic. The isothermal oxidation of the ZrB2-SiC-ZrC ceramic was carried out in static air at 1600±15 ºC. In the different oxidation stages, quantitative models were proposed for predicting oxidation kinetics.