The thermal oxidation of amorphous Cu–Zr alloys with different Cu:Zr ratios was studied by X-ray diffraction, Auger electron spectroscopy, and cross-sectional transmission electron microscopy. It was revealed that amorphous ZrO2/Cu-enriched bilayers are formed on the amorphous Cu–Zr alloys at 200–250 °C by the selective oxidation of Zr atoms in the alloys. Amorphous Cu-enriched particles (diameter: ~10 nm) are distributed randomly in the amorphous Cu-enriched sublayer of the am-Cu33at.%Zr67at.% alloy, due to the sluggish atomic diffusion in the amorphous Cu-enriched layer. In contrast, crystalline Cu-enriched particles (diameter: ~20 nm) are distributed densely in the amorphous Cu-enriched sublayer of the Cu50at.%Zr50at.% alloy. The different states of the Cu-enriched particles in the amorphous Cu-enriched sublayer of the am-Cu33at.%Zr67at.% and Cu50at.%Zr50at.% alloys are ascribed to thermodynamic reasons. The oxidation rate of amorphous Cu50at.%Zr50at.% alloy is faster than that of amorphous Cu33at.%Zr67at.% alloy, since the crystalline nanoparticles in the Cu-enriched sublayer provide fast migration pathways for ions, whereas the thick amorphous Cu-enriched sublayer presents as a diffusion barrier in the case of amorphous Cu33at.%Zr67at.% alloy.