The effect of various additives such as TiB2, HfB2, CrB2, TaB, TaSi2, LaB6, La2O3, and AlN on the oxidation behavior of ZrB2-based ultra-high-temperature ceramics (UHTCs) at 1800°C was investigated. The additions of CrB2 and HfB2 to the ZrB2–SiC composites have minor effect on the structure evolution of the oxide scale and oxidation resistance. However, all the other additives, especially for Ti and Ta compounds, have changed the oxidation mechanism resulting in a marked detrimental effect on the oxidation resistance of ZrB2-based UHTCs.