We present the details of epitaxial growth interface structure of single wurtzite AlN thin films on (111) Si substrates by laser-molecular-beam-epitaxy. High quality AlN thin films with atomically sharp interfaces can be obtained by Laser-MBE at a substrate temperature of 650 ±10°C. X-ray diffraction and high resolution transmission electron microscopy was used to study the details of epitaxial growth of AlN on Si(111) substrate. The orientation-relationship of AlN on Si(111) was studied from Si <110> and <112> zone axis and determined to be AlN [21 10]|Si and AlN |Si. The atomic structure of the interface was studied by high-resolution transmission electron microscopy and Fourier filtered image of cross-sectional AlN/Si(111) samples from both Si<110> and <112> zone axis. The results revealed the domain matching epitaxy of 4:5 ratio between the interplanar distances of Si(110) and AlN [21 10]. We also present similarities and differences between the growth mechanism of AlN/Si(111) and GaN/Si(111) heterostructures.