The preparation of large-area uniform and flat nanometer AlN film is of great significance for the manufacture of high-performance surface acoustic wave (SAW ) device.AlN thin films with preferential orientation (100) and uniform surface roughness have bee n deposited on Si (100) substrates by RF magnetron sputtering and the deposition process is opt imized.The results show that when the sputtering power is 120W,N₂:Ar=12∶8,the crystalline of AlN thi n film is the best and the surface roughness of (101.6mm)AlN thin film is from 3.03nm to 3.31nm,and t he average of surface roughness is 3.17nm.The results also show that the RF mag netron sputtering power and the nitrogen concentration are the important parameters for the preparation of l arge-area uniform and flat nanometer AlN films.