In reviewing the results reported for copper oxidation at intermediate temperatures from 573 to 1173 K, the oxidation mechanism at the lower part of this temperature range and the reason for the change in activation energy with decreasing the temperature remain unclear. To make it clear, copper oxidation is studied at 623–1073 K under 0.1 MPa O₂ using a commercial 99.9999% pure copper. The oxidation kinetics is essentially parabolic, and the activation energy decreases from 111 kJ/mol at 873–1073 K to 40 kJ/mol at 623–773 K . The growth of Cu₂O is predominant and it obeys the parabolic law at 623–773 K, as well as the case at 873–1073 K . In addition to grain boundary diffusion of copper along the fine and thin columnar Cu₂O grains, the non-protective CuO whisker layer, which cannot keep the oxygen potential constant at the Cu₂O/CuO interface, should be responsible for the decrease in the activation energy at 623–773 K.