The preferential orientation of aluminum nitride (AlN) thin films prepared by RF reactive magnetron sputtering method under the substrate temperature ranged from -20 ℃ to 20 ℃ have been studied. The crystal structure, surface roughness and the morphology of the films were analyzed by X ray diffractometry (XRD), atomic force microscope (AFM) and field emission scanning electron microscopy (FESEM). The results showed that when the substrate temperature was dropped to 0 ℃ or lower, the AlN (100) peak disappeared and the film favored the formation of oriented (002) preferential plane. The grain size and the surface roughness decreased as the decrease of substrate temperature. The film showed low surface roughness and optimal crystalline quality when the substrate temperature was at 0 ℃.