The authors report on the growth of GaN by nitrogen plasma-assisted molecular beam epitaxy (MBE) on a 2in. Si (111) substrates with a nanothick (∼4.8nm thick) γ-Al2O3 as a template/buffer. A thin layer of MBE-AlN ∼40nm thick was inserted prior to the growth of GaN. High-resolution transmission electron microscopy (HR-TEM) and high-resolution x-ray diffraction studies indicated that both of the nanothick γ-Al2O3 and AlN are a single crystal. Reflection high-energy electron diffraction, high-resolution x-ray scattering using synchrotron radiation, and cross-sectional HR-TEM measurements indicated an orientation relationship of GaN(0002)∥AlN(0002)∥γ-Al2O3(111)∥Si(111) and GaN[10−10]∥AlN[10−10]∥γ-Al2O3[2−1−1]∥Si[2−1−1]. A dislocation density of 5×(108–109)∕cm2 in the GaN ∼0.5μm thick was determined using cross-sectional TEM images under weak-beam dark-field conditions.