<p>Diamond/aluminum composites with excellent performance have broad application prospects in electronic packaging and heat dissipation. In order to improve the interfacial wettability between diamond and aluminum and to prevent excessive formation of Al₄C₃, silicon addition to the aluminum was used as the matrix. The composites with varying silicon content in the matrix were fabricated using the vacuum-assisted pressure infiltration method, and the effects of silicon content on microstructure and properties of the composites were systematically studied. Results show that the diamond/Al-Si composites exhibit a uniform and dense microstructure with good interfacial bonding. As the silicon content increases, both the thermal conductivity and bending strength of the composites first increase and then decrease, while the coefficient of thermal expansion decreases monotonically. When the silicon content in the aluminum matrix is 12wt.%, the composites reinforced with 180 µm diamond particles exhibit the highest thermal conductivity of 654 W·m⁻¹·K⁻¹, the highest bending strength of 296 MPa, and a remarkably low coefficient of thermal expansion of 5.96×10⁻⁶·K⁻¹. This study indicates that the silicon addition to the aluminum matrix can effectively suppress the excessive formation of Al₄C₃, and the coexistence of an appropriate amount of Al₄C₃ or SiC at the interface enhances interfacial bonding, thereby improving the performance of the composites.</p>