Abstract: The effects of BN content on the thermal conductivity, hardness and microstructure of AlN-BN composite ceramics prepared by ceramic injection molding (CIM) and pressureless sintering, as well as the rheological characteristics of the mixtures were discussed. The results showed that the mixtures had good fluidity and low temperature dependence of viscosity, suitable for ceramic injection molding. It was found that the thermal conductivity, relative density and hardness of the ceramics decreased with increasing BN content, which was attributed to the low conductivity and hardness of BN self and its card house structure generated during the sintering that can delay the densification process of the composite ceramics. In view of the demand on the thermal conductivity and hardness, a better content of BN in AlN-BN composite ceramics is thought to be from 10% to 15%, at which, the thermal conductivity of the composite ceramics is more than 120 W·m^-1·K^-1, the hardness in HRA is less than 80, and the relative density is more than 90%.