Y₂O₃ is regarded as one of the potential environmental barrier coating (EBC) materials for Al₂O_3f/Al₂O₃ ceramic matrix composites owing to its high melting point and close thermal expansion coefficient to Al₂O₃. However, the relatively high thermal conductivity and unsatisfactory calcium–magnesium–aluminosilicate (CMAS) resistance are the main obstacles for the practical application of Y₂O₃. In order to reduce the thermal conductivity and increase the CMAS resistance, four cubic bixbyite structured high-entropy oxides RE₂O₃, including (Eu_0.2Er_0.2Lu_0.2Y_0.2Yb_0.2)₂O₃, (Sm_0.2Er_0.2Lu_0.2Y_0.2Yb_0.2)₂O₃, (Sm_0.2Eu_0.2Er_0.2Y_0.2Yb_0.2)₂O₃, and (Sm_0.2Eu_0.2Lu_0.2Y_0.2Yb_0.2)₂O₃ were designed and synthesized, among which (Eu_0.2Er_0.2Lu_0.2Y_0.2Yb_0.2)₂O₃ and (Sm_0.2Er_0.2Lu_0.2Y_0.2Yb_0.2)₂O₃ bulks were prepared by spark plasma sintering (SPS) to investigate their mechanical and thermal properties as well as CMAS resistance. The mechanical properties of (Eu_0.2Er_0.2Lu_0.2Y_0.2Yb_0.2)₂O₃ and (Sm_0.2Er_0.2Lu_0.2Y_0.2Yb_0.2)₂O₃ are close to those of Y₂O₃ but become more brittle than Y₂O₃. The thermal conductivities of (Eu_0.2Er_0.2Lu_0.2Y_0.2Yb_0.2)₂O₃ and (Sm_0.2Er_0.2Lu_0.2Y_0.2Yb_0.2)₂O₃ (5.1 and 4.6 W·m^-1·K^-1) are only 23.8% and 21.5% respectively of that of Y₂O₃ (21.4 W·m^-1·K^-1), while their thermal expansion coefficients are close to those of Y₂O₃ and Al₂O₃. Most importantly, HE RE₂O₃ ceramics exhibit good CMAS resistance. After being attacked by CMAS at 1350 ℃ for 4 h, the HE RE₂O₃ ceramics maintain their original morphologies without forming pores or cracks, making them promising as EBC materials for Al₂O_3f/Al₂O₃ composites.