A considerable difference in the diffusion behavior of copper was found between epitaxial GaAs and bulk GaAs. The carrier concentration of epitaxial GaAs scarcely decreased (much less than 1×1015cm−3) with copper diffusion at around 500°C, while carrier concentration of undoped bulk GaAs decreased about (3–5)×1016cm−3with the diffusion under the same conditions. The diffused‐copper concentration in the epitaxial GaAs, measured by radioactive copper Cu64, was less than that in the bulk GaAs by more than one order of magnitude. These differences in the copper diffusion can be understood by the difference in the amount of defects, and therefore, by the difference in solubility of substitutional copper between epitaxial and bulk GaAs.