Abstract: Diamond/Al composites were fabricated by vacuum pressure infiltration method. The thermal properties of the composites with different particle sizes and grades were investigated. The results show that the thermal expansion coefficient of the composite decreased with the decrease of particle size of common grinded diamond when the volumn fraction of diamond did not change. The thermal expansion coefficient of the composite made of MBD4 grade diamonds was 6.8×10-6K-1, which was the lowest among all the composites, and the thermal conductivity of the composite was the highest. The phenomenon of selective adhesion between MBD4 grand diamond and aluminum matrix was observed, and (100) face of the diamond was inclined to combine with the pure aluminum.