C-axis-oriented aluminum nitride (AlN) films were successfully prepared on a Y-128 degrees LiNbO3 substrate by rf magnetron sputtering. The dependence between sputtering pressures and the physical structures of the films (crystalline structure and micromorphology) was investigated by x-ray diffraction (XRD) and atomic force microscopy (AFM). The result showed that the higher c-axis-oriented AlN films appeared at lower sputtering pressures and the uneven grain structures existed as multioriented XRD patterns appeared in the high sputtering pressure region (5-9 mTorr). The best orientation and microstructure of the films was prepared at 3 mTorr.