Aluminium nitride thin films were grown on Si(111) substrates by ion beam assisted deposition. Epitaxial growth of the AlN films was obtained at the low temperature of 450 degrees C. Characterization of the AlN epitaxial layer was carried out by grazing-incidence diffraction (GID) and high resolution transmission electron microscopy (HRTEM). GID analysis showed two kinds of epitaxial orientation between AlN epitaxial layer and Si substrate. An amorphous layer was observed at the interface between AlN film and Si by cross-sectional HRTEM analysis. The amorphous layer could release the misfit strain between the AlN film and the Si substrate in an early stage of growth. The AlN epitaxial layer formed despite the large lattice mismatch. In addition, the optical properties of AlN films including transmittance, energy band gap, and refractive index were determined and discussed.