Aluminum nitride (AlN) films withcaxis orientation were grown on (111) Si substrates by laser molecular beam epitaxy. The current transport mechanisms and the density of interface state of Au/AlN/Si MIS structure were studied. The AlN/Si heterojunctions exhibited a typical rectifying characteristic, which was attributed to the spacechargelimited current mechanism. The ideality factor was 2.88 and the interface state density of AlN/Si was about 1.1×1012eV-1·cm-2, mainly distributed in 0.26 eV distancing the top of valence band of Si, and consisted of O impurities, N vacancy and AlSi bond formed from Si atoms in place of N atoms during the AlN films growth.