The oxidation failure of a copper alloy lead frame with/without a copper plating layer was investigated. The oxidation rate and adhesion strength of oxide films on copper alloy substrates were studied by measuring the thickness and by carrying out peel tests. The adhesion strength of the oxide film was mainly influenced by the composition but not the thickness of the oxide film. The highest adhesion strength was obtained when the oxide film was composed mainly of Cu2O. When the thickness of the copper preplated layer was over 0.165 mu m, the Cu atoms of the preplated copper were available for oxidation. Thus the oxidation process was within the copper preplated layer, and the main product of the oxidation was Cu2O. It was found that the large column grain of the oxide film on the copper alloy with a copper plated layer, favored the diffusion of copper or oxygen atoms that led to the formation of Cu2O, and lead to higher adhesion strength. This indicated that the oxidation resistance of a copper alloy lead frame can be effectively improved by electroplating copper.