The mechanical properties of B/Al composite were measured at room temperature before and after thermal-mechanical cycling (TMC) in the temperature interval from –125°C to 125°C under constant stress of 30 MPa. The effects of TMC on microstructure and tensile fracture behavior of B/Al composite were studied by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The fiber / matrix interfaces were degraded during TMC, the extent of which was enhanced with increasing the cycles, causing a measurable decrease of stageⅠmodulus of the B/Al composite. The TMC induced the dislocation generation in the aluminum matrix and the dislocation density increased with increasing the cycles. The tensile strength of the composites increased with the cycles in the early stage of TMC, but subsequently decreased after further TMC. The interfaces in the B/Al composite changed from the strongly-bonded toward the appropriately-bonded, and then to the weakly–bonded ones with increasing the cycles.