Hexagonal gallium nitride (h-GaN) films of about 400 nm are grown by using radio frequency (RF) plasma source assisted molecular beam epitaxy on (0 0 1)-oriented GaAs substrate. Before the growth of GaN epilayer an A1As layer of about 10 nm was grown at 700 degrees C and nitridized with the temperature ramping from 540 to 730 degrees C. Then low-temperature GaN or AIN was grown as the buffer layer. The epilayers were characterized by using high-resolution X-ray diffraction (HRXRD), atomic force microscopy (AFM), and Raman scattering. By using AIN instead of GaN as the buffer layer, the GaN epilayer quality and morphology are significantly improved.