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In this study, fine Y2O3–MgO composite nanopowders were synthesized via the sol–gel method. Dense Y2O3–MgO composite ceramics were fabricated by pre‐sintering the green body in air at different temperatures for 1 h and then subjecting the sintered bodies to hot isostatic pressing at 1300°C for 1 h. The effects of pre‐sintering temperature on the microstructural, mechanical, and optical properties of the resulting ceramics were studied. The average grain size of the ceramics was increased, whereas their hardness and fracture toughness were decreased with increasing pre‐sintering temperature. A maximum fracture toughness of 1.42 MPa·m1/2 and Vickers hardness of 10.4 GPa were obtained. The average flexural strength of the ceramics was 411 MPa at room temperature and reached 361 MPa at 600°C. A transmittance of 84% in the 3–5 µm region was obtained when the composite ceramics were sintered at 1400°C. Moreover, a transmittance of 76% in the 3–5 µm region was obtained at 500°C. [ABSTRACT FROM AUTHOR]