Abstract: Aluminum nitride ceramics are important heat-dissipation materials for high-power electronic devices. However, the high sintering temperatures required by conventional processing routes limit their practical application and increase fabrication costs. It is therefore necessary to develop preparation methods capable of achieving densification at relatively low temperatures. To address the difficulty of simultaneously obtaining high densification and high thermal conductivity in polycrystalline AlN ceramics under reduced-temperature sintering conditions, this work adopts a stepwise research strategy. First, the densification behavior and thermal conductivity of pure AlN under high-pressure assistance were investigated, and the optimal sintering conditions were identified. Under additive-free conditions, dense pure AlN ceramics with clean grain boundaries were fabricated at 5.0 GPa and 1400 °C, exhibiting a thermal conductivity of 101.6 W·m ^-1·K^-1. Based on these optimized conditions, the AlN/diamond composite system was further studied, and the effect of diamond volume fraction on the structure and properties of the composites was systematically examined. The results show that the thermal conductivity of the composites first decreases and then increases with increasing diamond content, reaching 112.4 W·m^-1·K^-1 at 33.3 vol%. Mechanistic analysis indicates that interfacial thermal resistance dominates at low diamond contents, whereas at high diamond contents the enhancement of heat transport by thermally conductive diamond pathways becomes more significant. By taking full advantage of the processing benefits of high-temperature and high-pressure technology, this work achieves substantial improvement of AlN-based materials at temperatures lower than those required in conventional sintering, thereby providing a new route for the low-temperature fabrication of high-performance thermally conductive ceramics.