Two powder mixing processes, mechanical mixing (MM) and mechanical alloying (MA), were used to prepare mixed Al/diamond powders, which were subsequently consolidated using spark plasma sintering (SPS) to produce bulk Al/diamond composites. The effects of the powder mixing process on the morphologies of the mixed powders, the microstructure and the thermal conductivity of the composites were investigated. The results show that the powder mixing process can significantly affect the microstructure and the thermal conductivity of the composites. Agglomerations of the particles occurred in mixed powders using MM for 30 min, which led to high pore content and weak interfacial bonding in the composites and resulted in low relative density and low thermal conductivity for the composites. Mixed powders of homogeneous distribution of diamond particles could be obtained using MA for 10 min and MM for 2 h. The composite prepared through MA indicated a high relative density but low thermal conductivity due to its defects, such as damaged particles, Fe impurity, and local interfacial debonding, which were mainly introduced in the MA process. In contrast, the composite made by MM for 2 h demonstrated high relative density and an excellent thermal conductivity of 325 W·m-1·K-1, owing to its having few defects and strong interfacial bonding. Two powder mixing processes, mechanical mixing (MM) and mechanical alloying (MA), were used to prepare mixed Al / diamond powders, which were used consolidated by spark plasma sintering (SPS) to produce bulk Al / powder mixing process on the morphologies of the mixed powders, the microstructure and the thermal conductivity of the composites were investigated. The results show that the powder mixing process can significantly affect the microstructure and the thermal conductivity of the composites. mixed powders using MM for 30 min, which led to high pore content and weak interfacial bonding in the composites and resulted in low relative density and low thermal conductivity for the composites. Mixed powders of homogeneous distribution of diamond particles could be obtained using MA for 10 min and MM for 2 h. The composite prepared through MA indicated a high relative density but low thermal conducti vity due to its defects, such as damaged particles, Fe impurity, and local interfacial debonding, which were primarily introduced in the MA process. In contrast, the composite made by MM for 2 h demonstrated high relative density and an excellent thermal conductivity of 325 W · m-1 · K-1, owing to having defects and strong interfacial bonding.