For obtaining materials with high thermal conductivities and suitable thermal expansion coefficient for thermal management applications, diamond/Al composites were fabricated by the low-cost pressureless metal infiltration. The resulting composites exhibited thermal conductivities as high as 518.7 W/m•K and thermal expansion coefficient as low as 4.61×10-6/K friendly matching with semiconductors materials like Si or GaAs. The diamond particles were not only well embedded, but also uniformly distributed in the metallic matrix along with SEM observations of the composites. Fractograph of the composites illustrated that aluminum matrix fracture was the dominant fracture mechanism and the stepped breakage of diamond particles indicated strong interfacial bonding between diamond and the Al matrix. The Si skeleton with coralline morphology in the interface between diamond and the matrix were found to play a role of bridge in the interfacial structure and result in distinctive interfacial bonding. Also a little content of Al4C3 were realized to have positive effect on the improved thermal conductivities for promoting interfacial bonding between aluminum and diamond. In addition, the excellent mechanical behavior of the composite was illustrated. The results shows a superior Young’s modulus of 286 GPa compared with traditional thermal management materials and relatively high bending strength of 306MPa.