High-quality single domain GaAs epitaxial layers were successfully grown on (100)Ge substrates. Growth was carried out using conventional metalorganic chemical vapor deposition at atmospheric pressure. Antiphase domain free GaAs epitaxial layers were obtained by thermal etching of the Ge surface just prior to growth. Mosaic surface morphology and antiphase boundaries characteristic of domain structures were completely absent in epi-layers following thermal etching. Photoluminescence revealed that domain free epi-layers exhibited characteristics comparable to those of GaAs homoepitaxial layers.