Aluminum nitride films have been synthesized by reactive magnetron sputtering on n-Si(100) substrates. AlN layers with thicknesses from 2 to 150 nm were obtained to establish a correlation between the structure of the films and their electrical conductivity. Electron microscopy revealed that the amorphous structure of the films passes to nanocrystalline one while moving away from the substrate surface. Films with thicknesses below 20 nm had a high conductivity: up to 10 (Ω cm)–1; with an increase in thickness the conductivity dropped to 10–7 (Ω cm)–1. The high conductivity of thin AlN layers is believed to be due to the high density of the boundaries of grains built-in into amorphous matrix.