Mechanical properties of the B/A1 composites exposed in hot air up to 300℃C,.500＄ are investigated.Experimental results show that thermal treatment at 300℃ may result in less decrease with tensile strength remained about 72% ,and ultimate strain remained 74% after 100 h ,for example ,but thermal treatment at 500℃ only for 5 h can significantly decrease the tensile strength of the B/Al composites.Failure mode of the composite also changes from cumulative to typical noncumulative after long time exposure at 500℃.Interfacial microstructure and phase composition of the B/Al composite before and after exposure is studied by TEM and SEM.It is showed that interfacial reaction is the main reason of decrease ofmechanical properties of the B/M composites.