Abstract: AlON-AlN composite ceramics were prepared with Y2O3 as sintering additive by pressureless reaction sintering in nitrogen atmosphere.The phase compositions and microstructure of AlON-AlN composite ceramics were characterized by XRD and SEM.The effects of sintering temperature and Al2O3 and Y2O3 content on sintering performance,mechanical properties and thermal conductivity were studied.The results show that the AlON-AlN composite ceramics could be fabricated at 1 650-1 800 ℃.As the temperature increased,the bend strength and thermal conductivity both increased first and then decreased,and the maximum values(378 MPa,38 W·m-1·K-1) were gained respectively at 1 750 ℃ and 1 700 ℃.As the Al2O3 content increased,the bend strength of the sample increased till Al2O3 content up to 30%,and then decreased.The thermal conductivity decreased obviously with the increase of Al2O3 content.The bend strength increased remarkably as the Y2O3 content increased while the maximum of thermal conductivity appeared when the Y2O3 content was 3%.