Microstructure of hot-pressed Al2O3/Cu composites with a different temperature for atmosphere switching from H-2 to Ar has been studied. When the composite atmosphere was switched at 1100degreesC it lead to a more homogeneous microstructure and the highest fracture strength with smaller deviation than when the atmosphere was switched at 1450degreesC. The strong sensitivity to atmosphere, especially the oxygen content in the atmosphere, was found to be responsible for observed changes, based upon the interfacial phenomena related to the formation of CuAlO2.