The microstructural evolution and performance of diamond/Al composites during thermal cycling has rarely been investigated. In the present work, the thermal stability of diamond/Al composites during thermal cycling for up to 200 cycles was explored. Specifically, the thermal conductivity (λ) of the composites was measured and scanning electron microscopy of specific areas in the same samples was carried out to achieve quasi-in situ observations. The interface between the (100) plane of diamond and the Al matrix was well bonded with a zigzag morphology and abundant needle-like Al4C3 phases. By contrast, the interface between the (111) plane of diamond and the Al matrix showed weak bonding and debonded during thermal cycling. The debonding length increased rapidly over the first 100 thermal cycles and then in-creased slowly in the following 100 cycles. The λ of the diamond/Al composites decreased abruptly over the initial 20 cycles, increased after-ward, and then decreased monotonously once more with increasing number of thermal cycles. Decreases in the λ of the Al matrix and the cor-responding stress concentration at the diamond/Al interface caused by thermal mismatch, rather than interfacial debonding, may be the main factors influencing the decrease in λ of the diamond/Al composites, especially in the initial stages of thermal cycling.