GaAs layers were grown on Si (001) substrates by molecular beam epitaxy (MBE) and migration enhanced epitaxy (MEE). They were examined by transmission electron microscopy, doublecrystal X-ray diffraction and Rutherford backscattering/channeling technique. Initial layer growth in both MEE and MBE was governed by three-dimensional nucleation but a stronger tendency for GaAs islands to align along the surface steps of Si was observed in the case of MEE. There was no measurable tilt between the (001) planes of GaAs and Si if growth was initiated by MEE at low temperature, prior to further growth by MBE at higher temperatures. On the contrary, the tilt angle was 0.34• when the entire structure was prepared by conventional two-step MBE. Rutherford backscattering measurements indicated a significant reduction in the density of defects throughout MEE/MBE GaAs in comparison with MBE GaAs.