Abstract: The effects of the amount of Si added to Al-8Mg alloy on the microstructure and thermal conductivity of pressureless infiltrated SiC_p/Al composites were investigated. The results show that Si addition to matrix can reduce the porosity of the infiltrated composites by the improvement of SiC/Al wettability, suppress the SiC/Al interracial reaction and increase the relative density of the infiltrated composites. Without Si addition to At-SMg alloy, the composites exhibited a lower thermal conductivity and relative density because of poor wettability and severe interracial reaction. With increasing the Si content to 12%, the interracial reaction was suppressed and the thermal conductivity of the composites is the maximum. With further increasing Si content, the thermal conductivity of the composites decreased because the thermal conductivity of matrix itself decreased.