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Abstract Polymer‐derived amorphous SiCN has excellent high‐temperature stability and properties. To reduce the shrinkage during pyrolysis and to improve the high‐temperature oxidation resistance, Y₂O₃was added as a filler. In this study, polymer‐derived SiCN–Y₂O₃composites were fabricated by mixing a polymeric precursor of SiCN with Y₂O₃submicron powders in different ratios. The mixtures were cross‐linked and pyrolyzed in argon. SiCN–Y₂O₃composites were processed using field‐assisted sintering technology at 1350°C for 5 min under vacuum. Dense SiCN–Y₂O₃composite pellets were successfully made with relative density higher than 98% and homogeneous microstructure. Due to low temperature and short time of the heat‐treatment, the grain growth of Y₂O₃was substantially inhibited. The Y₂O₃grain size was ∼1 μm after sintering. The composites’ heat capacity, thermal diffusivity, and thermal expansion coefficients were characterized as a function of temperature. The thermal conductivity of the composites ceramics decreased as the amount of amorphous SiCN increased and the coefficient of thermal expansion (CTE) of the composites increased with Y₂O₃content. However, the thermal conductivity and CTE did not follow the rule of mixture. This is likely due to the partial oxidation of SiCN and the resultant impurity phases such as Y₂SiO₅, Y₂Si₂O₇, and Y_4.67(SiO₄)₃O.