The intermediate nucleation layer effects on the crystal structure of GaN epitaxial layers grown on GaAs (0 0 1) substrates by solid-sourer molecular beam epitaxy using RF-N-2 plasma as a nitrogen source were investigated. The crystal structure of GaN grown on (0 0 1) GaAs substrates was critically influenced by the nucleation layer, that is, mainly cubic GaN was grown directly on the GaAs substrate with the epitaxial relationship of GaN (0 0 1)//GaAs(0 0 1) and GaN[1 1 0]//GaAs[1 1 0], while hexagonal GaN was frown on a very thin AlAs intermediate layer with the epitaxial relationship of GaN(0 0 0 1)//GaAs(0 0 1) and GaN[1 1 (2) over bar 0]//GaAs[1 1 0]. X-ray diffraction and transmission-electron-microscope are used to analyze the crystal structure of the two kinds of epilayers.