jats:titleAbstract</jats:title>jats:pA novel methodology combining multiscale mechanical testing and finite element modeling is proposed to quantify the sintering temperature‐dependent mechanical properties of oxide matrix composites, like aluminosilicate (AS) fiber reinforced Aljats:sub2</jats:sub>Ojats:sub3</jats:sub> matrix (ASjats:subf</jats:sub>/Aljats:sub2</jats:sub>Ojats:sub3</jats:sub>) composite in this work. The results showed a high‐temperature sensitivity in the modulus/strength of AS fiber and Aljats:sub2</jats:sub>Ojats:sub3</jats:sub> matrix due to their phase transitions at 1200°C, as revealed by instrumented nanoindentation technique. The interfacial strength, as measured by a novel fiber push‐in technique, was also temperature‐dependent. Specially at 1200°C, an interfacial phase reaction was observed, which bonded the interface tightly, as a result, the interfacial shear strength was up to ≈450 MPa. Employing the measured micro‐mechanical parameters of the composite constituents enabled the prediction of deformation mechanism of the composite in microscale, which suggested a dominant role of interface on the ductile/brittle behavior of the composite in tension and shear. Accordingly, the ASjats:subf</jats:sub>/Aljats:sub2</jats:sub>Ojats:sub3</jats:sub> composite exhibited a ductile‐to‐brittle transition as the sintering temperature increased from 800 to 1200°C, due to the prohibition of interfacial debonding at higher temperatures, in good agreement with numerical predictions. The proposed multiscale methodology provides a powerful tool to study the mechanical properties of oxide matrix composites qualitatively and quantitatively.</jats:p>