GaN films of about 300nm were grown on GaAs(001) substrates by molecular beam epitaxy (MBE).X-ray double-crystal diffraction and room-temperature photoluminescence measurements show that the films grown on nitridized GaAs nucleation layer are pure cubic GaN while the films grown on nitridized AlAs nucleation layer are pure hexagonal GaN.The present study shows that the phase of GaN samples grown on GaAs substrates can be controlled using different nucleation layers