Nucleation and growth of wurtzite AlN layers on nominal and off-axis Si(001) 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. For the films grown on the nominal Si(001) surface, cross-sectional transmission electron microscopy and X-ray diffraction investigations revealed a two-domain film structure (AlN 1 and AlN 2 ) with an epitaxial orientation relationship of [0001] AlN || [001] Si and 〈01 1 ̄0〉AlN 1 || 〈 2 ̄110〉AlN 2 || [110] Si . The epitaxial growth of single crystalline wurtzite AlN thin films has been achieved on off-axis Si(001) substrates with an epitaxial orientation relationship of [0001] AlN parallel to the surface normal and 〈0110〉 AlN || [110] Si .