GaN heteroepitaxial growth on sapphire (0 0 0 1) substrates was carried out by radio-frequency (RF) plasma-assisted molecular beam epitaxy. The lattice polarity of RF-MBE-grown GaN films on sapphire (0 0 0 1) substrates was investigated as a function of buffer layer process. GaN films with Ga-face lattice polarity were fabricated using an AlN high-temperature buffer layer before GaN growth. Direct measurements by coaxial impact collision ion scattering spectroscopy, as well as reflection high-energy electron diffraction, scanning electron microscope observations of surface morphologies and chemical etching, confirmed the polarity assignment. Realization of Ga-polarity films on sapphire substrates by RF-MBE is a promising result and is expected to lead to a breakthrough in fabricating high-quality MBE-grown III-nitride films for device applications.