Hexagonal GaN film has been grown on (0 0 1) GaAs substrate with nitridized AlAs buffer layer. The (0 0 0 1) plane of the hexagonal GaN(h-GaN) is parallel to the (0 0 1) plane of the GaAs substrate. The nitridized AlAs/GaAs(0 0 1) material was analyzed by high-resolution transmission electron microscopy (HRTEM) and atomic force microscopy (AFM). The HRTEM observations show that hexagonal AlN(h-AlN) can nucleate on the AlAs surface after nitridation with its (0 0 0 1) plane parallel to the (0 0 1) plane of the AlAs. Some cubic AlN(c-AlN) micro grains were also observed. The close-packed atom planes of the c-AlN grains are parallel to the (0 0 1) planes of the AlAs. The AFM result shows that the surface morphology of the nitridized AlAs is rough with high-density dolts. Such AIN layer can be takers as a prelayer for growing h-GaN on the (0 0 1) GaAs substrate.