Interfacial reaction during thermal exposure of the Al₂O₃/Al-alloy monofilament composites has been investigated in relation to tensile properties of the composites. In the case of the Al₂O₃/pure-Al composites, reaction products such as Al₂O₃and Si were formed at interfaces due to reaction between SiO₂in the fiber and Al matrix at 600℃, while the interfacial reaction did not occur at 500℃. In the case of the Al₂O₃/7075-alloy composites, reaction products such as MgO, Al₂O₃and Si began to form at 500℃, and they became remarkably coarse at 600℃. Tensile strength and the Weibull moduli of the thermally exposed composites decreased with the progress of interfacial reaction. Tensile strength and the Wibull moduli of the Al₂O₃/pure-al composites were much higher than those of the Al₂O₃/7475-alloy composites, respectively, when they were exposed at 600℃.