An investigation of the initial growth mechanism of cubic GaN film during the nitridation of an AlGaAs buffer layer and the initial growth stage of an epilayer by RF-plasma assisted molecular beam epitaxy was carried out using in situ reflection high energy electron diffraction (RHEED), and high resolution X-ray diffraction (HRXRD). It was found that hexagonal GaN nuclei grew on (1 1 1) facet during nitridation of AlGaAs buffer layer, but a highly pure cubic-phase GaN epilayer was grown on the nitrided AlGaAs buffer layer.