Hexagonal AlN thin films have been deposited by DC reactive magnetron sputtering at room temperature. For a first set of samples, sputtered AlN films were deposited on silicon Si (100) substrates. For a second set, AlN films were deposited on 200 nm (002) oriented AlN epitaxial layer obtained by Molecular Beam Epitaxy (MBE) on Si (111). X-ray Diffraction (XRD) and High Resolution Transmission Electron Microscopy (HRTEM) analysis of the synthesized films on Si (100) substrate have shown an amorphous phase close to the interface followed by a nano-crystalline layer exhibiting (100) and (002) orientations of the hexagonal AlN crystalline phase. Finally a relatively well crystallised layer with a single (002) orientation has been observed for the thickest films. This improvement of crystalline quality with film thickness has been consistent with a drastic decrease of the films stress from –1.2 GPa at 300 nm to no stress around 800 nm and even 0.3 GPa tensile stress for 1.5 μm thick film. This behaviour was different when epitaxial AlN was used as substrate. In fact, we have observed thanks to HRTEM images and Selected Area Electron Diffraction (SAED) patterns, that the AlN film deposited on such a substrate exhibits the same crystalline quality and have the same orientation as the AlN epitaxial layer during the first 500 nm of thickness. A further increase of film thickness has caused a decrease on the crystalline quality. The films became polycrystalline while preserving a (002) preferential orientation.