Highly c-axis-oriented aluminum nitride (AIN) thin films were prepared on polycrystalline Si3N4 and SiC substrates by helicon plasma sputtering. The difference in the substrate materials scarcely influenced the crystal structures of the films. The full width at half-maximum (FWHM) of the X-ray rocking curves of the films was 3.2 degrees, which is the smallest value for AIN thin films deposited on polycrystalline substrates to our knowledge. Annealing at 800 degreesC in vacuum decreased the FWHM from 3.2 degrees to 2.8 degrees. The structure of the films was densely packed and consisted of many fibrous grains. The films developed c-axis orientation on the polycrystalline substrates, because the interaction between the films and substrate surfaces was small, and (100) and (110) AIN planes grew preferentially. The films were piezoelectric and generated electrical signals in response to mechanical stress.