The annealing of sputtered AlN films with different thicknesses grown on sapphire in nitrogen ambient was investigated. In the annealing, two AlN films on sapphire were overlapped "face-to-face" to suppress the thermal decomposition of the AlN films. The sputtered AlN films with small grains consisted of columnar structure were initially aligned with (0002) orientation but became slightly inclined with increasing film thickness resulting in the formation of a two-layer structure. After annealing, films became a single crystalline layer regardless of the film thickness, and their crystallinity markedly improved after annealing at 1600-1700 degrees C. The full widths at half maximum of the (0002)- and (1012)-plane X-ray rocking curves were improved to 49 and 287 arcsec, respectively, owing to the annihilation of domain boundaries in the sputtered AlN films, which concurrently increased the compressive stress in the films. (C) 2016 The Authors. Published by Elsevier B.V.