Nanocrystalline aluminum nitride films (70-1400 nn thick) were prepared by rf-magnetron sputtering from an Al-target at a substrate temperature of 540 K. For the first time, the influence of the pressure on the film properties was investigated for sputter-deposition in pure nitrogen-atmosphere. The pressure was varied in the range from 0.15 to 4.0 Pa. The deposition rate decreases monotonically from 0.35 to 0.17 nm/s with pressure. Films of 300 nm thickness show at 1 Pa nitrogen pressure a maximum of the crystallite size (in direction parallel to the film normal), which amounts to 30 nm. Highly strained crystallites grow at pressures below 0.5 Pa. The films are (excepted sputtering above 2 Pa) in general highly c-axis oriented. It is found that the crystallites are of a cylinder-like shape with axis orientation parallel to the film normal and a roughly constant length-to-diameter ratio of 4. The optical properties static refractive index and optical Tauc gap depend weakly on the pressure and are on average 2.0 and 5.0 eV, respectively. The bombardment of the growing AlN by energetic nitrogen neutrals originating from reflection at the target, which was simulated by TRIM.SP Monte-Carlo calculations, is suggested to be the general origin of the preferred c-axis growth of sputter-deposited films.