The epitaxial growth of crystalline wurtzite AlN thin films on (001) Si substrates by plasma-assisted molecular-beam epitaxy is reported. The nucleation and the growth dynamics have been studied in situ by reflection high-energy electron diffraction. Cross-sectional transmission electron microscopy and x-ray diffraction investigations revealed a two-domain film structure (AlN1 and AlN2) with a 30° rotation between neighboring domain orientations and an epitaxial orientation relationship of [0001]AlN∥[001]Si and 〈011̄0〉AlN1∥〈2̄110〉AlN2∥[110]Si. A model for the nucleation and growth mechanism of 2H–AlN layers on Si(001) is proposed.