Amorphous SiBCN powders were prepared by high-energy ball milling-mechanical alloying method. The SiBCN/HfC ceramic composites were consolidated by spark plasma sintering (SPS). The influence of high temperature heat treatment on the microstructural evolution and phase composition was investigated. The results showed that the oxygen was introduced during the mechanical alloying process, leading to the oxidation of BN to form B₂O_3. HfO₂ was formed in the sintering process result from HfC oxydation and reduced to HfB₂ by carbothermal reduction reaction after heat-treatment at 1600 ℃ for 1 h. Both HfO₂ and HfC were reduced to HfB₂ during the heat-treatment at 1650 ℃ and 1800 ℃ by reaction of HfC + C + B₂O₃ → HfB₂ + CO. Introduction of oxygen causes phase transformation of the SiBCN/HfC ceramic composites after heat treatment at high temperatures, during which ceramic matrix becomes loose and porous due to volatilization of the gaseous byproducts. Therefore, control of oxygen content is the key to the real applications of SiBCN/HfC ceramic composites at high temperatures.