Diamond/aluminium composites have attracted attention in the field of thermal management of electronic packaging for their excellent properties. In order to solve the interfacial problem between diamond and aluminium, a novel process combining pressure infiltration with vacuum-assisted technology was proposed to prepare diamond/aluminum composites. The effect of diamond particle size on the microstructure and properties of the diamond/Al-12Si composites was investigated. The results show that the diamond/Al-12Si composites exhibit high relative density and a uniform microstructure. Both thermal conductivity and coefficient of thermal expansion increase with increasing particle size, while the bending strength exhibits the opposite trend. When the average diamond particle size increases from 45 μm to 425 μm, the thermal conductivity of the composites increases from 455 W·m^-1·K^-1 to 713 W·m^-1·K^-1 and the coefficient of thermal expansion increases from 4.97×10-6 K^-1 to 6.72×10-6 K^-1, while the bending strength decreases from 353 MPa to 246 MPa. This research demonstrates that high-quality composites can be prepared by the vacuum-assisted pressure infiltration process and the thermal conductivity of the composites can be effectively improved by increasing the diamond particle size.