A nano dual-phase powder with great sinterability was synthesized by molten-salt assisted borothermal reductions at 1100 °C using B, ZrO2, HfO2, Ta2O5, Nb2O5 and TiO2 powders as raw materials. Single-phase (Zr0.2Hf0.2Ta0.2Nb0.2Ti0.2)B2 high-entropy ceramic was prepared by spark plasma sintering using the as-synthesized nano dual-phase powder. Oxidation behavior of the (Zr0.2Hf0.2Ta0.2Nb0.2Ti0.2)B2 ceramic was investigated over the range of 30–1400 °C in air and the result indicated that the rapid oxidation of ceramic began at 1300 °C. The phenomenon could be ascribed to the rapid volatilization of B2O3 from oxide scale. A layered structure was formed at the cross section of (Zr0.2Hf0.2Ta0.2Nb0.2Ti0.2)B2 ceramic after oxidation. The relationship between partial pressures of gaseous metal oxides and oxygen partial pressures was calculated, which inferred that the formation of layered structure could be ascribed to the active oxidation of (Zr0.2Hf0.2Ta0.2Nb0.2Ti0.2)B2, the generation of gaseous metal oxides, their outward diffusion and further oxidation.