Abstract: Diamond/Al, diamond/AlSi7 and diamond/AlSi9 composites were fabricated by using gas pressure infiltration, and the decline behaviors for the thermal conductivity of the composites when exposed to the air were analyzed. The results show that the reaction product Al₄C₃ between the diamond and the molten Al can decompose into Al(OH)₃ in the humid air, resulting in the decline for the thermal conductivity of diamond/Al composites. Adding element Si into Al is found to inhibit the decline rate effectively, the mechanism of which are as follows:decrease in the amount of Al₄C₃ because of the decrease in the solubility of C of diamond in molten Al and the preferential precipitation of eutectic Si on diamond; increase in the density of the diamond/AlSi composites to prevent the contact between Al₄C₃ and moisture. Some methods of inhibiting the decline of the thermal conductivity of diamond/Al composites were also discussed, which is hopeful to prolong the service time of the composites in humid environment.