One limitation of high-entropy ceramics is the low toughness that could hinder their implementations in the relevant industries despite the diverse potential applications. This challenge is increasingly attempted in recent years with high-entropy ceramic composites which consisted of high-entropy ceramic matrix and secondary phase. A suitable secondary phase candidate is Al2O3 which to-date has not been reported for such ceramic composites. In this work, bi-phase high-entropy ceramic composites of cubic fluorite oxide matrix and Al2O3 secondary phase were in situ synthesized from HfO2, ZrO2, CeO2, CaO and Al2O3 nano powders via solid-state reaction method. Variation of thermal and mechanical properties of the composites with Al ratio was due to the combined effects of the inherent properties of Al2O3, concentrations of grain boundary and of oxygen vacancies. The high-entropy ceramic composites enhanced by Al2O3 were also compared with yttria-stabilised zirconia for potential application as thermal barrier coating material.