When aluminum nitride (AIN) thin films are applied to surface acoustic wave devices, (100) orientated films are necessary, because the electromechanical coupling coefficient of the (100) oriented films are superior to that of the (001) oriented films. However, it is hard to prepare the (100) oriented films by sputtering and ion plating. In this study, preferential orientation of AlN films was investigated. It was found that the (100) oriented films were deposited by the reactive DC magnetron sputtering method at a long distance (L) between the Al target and substrate (above 120 mm) and at high sputtering pressures (above 0.6 Pa). According to our theory, the relative growth rate of (100)/(001) is varied by changing the deposition unit from atoms to a dimer such as Al-N. Namely, when the mean free path of Al and N is longer than the distance L, Al and N atoms deposit directly on the substrate and the (001) oriented films grow. When it is shorter than the distance L, the collisions of Al and N atoms occur in the space between the target and the substrate and the Al-N dimers are formed and deposit on the substrate. In this case, the (100) orientation is enhanced.