Interfaces of the Al18B4O33w/Al composite subjected to different thermal exposure treatments were investigated by means of TEM and its tensile properties were tested. The results of observations indicated that the interfacial reaction took place in the specimens thermally exposed at 500 degreesC for 50h, the products of which were gamma -Al2O3 with the spinel structure and boron. In addition, tensile tests showed that Yong's modulus, ultimate tensile strength as well as yield strength initially increased, then decreased with time at 500 degreesC; however, Young's modulus and ultimate tensile strength both had an increasing tendency with temperature for a given 10 h exposure time, but yield strength initially increased and then decreased with increasing temperature for a 10h exposure time. Before the interfacial reaction took place, welding of the microcracks at interfaces of the starting as-squeeze cast composite was responsible for an improvement in tensile properties. However. the interfacial reaction always deteriorated the tensile properties of the Al18B4O33w/Al composite.