Epitaxial GaAs layers have been grown on composite CaF//2/Si structures by molecular beam epitaxy. Fundamental growth characteristics of GaAs films on (100) and (111) oriented substrates have been investigated. GaAs films were found to grow epitaxially at conventional growth temperatures (600 degree C). Epitaxial growth of GaAs on (100) substrates was observed at lower temperatures than the growth on (111) substrates. But the crystalline quality of GaAs films grown on (111) substrates was superior to that of the films grown on (100) substrates. GaAs films which had the quality comparable with a bulk GaAs crystal, as measured by ion channeling, were obtained on (111) substrates, though the surface morphology of the films was not satisfactory.