c-axis oriented aluminum nitride (AlN) films deposited on GaAs substrates by reactive rf magnetron sputtering were fabricated. The structural and morphological characterizations were found to be sensitive to the deposition conditions including rf power, Nz concentration, sputtering pressure and substrate temperature. A highly c-axis oriented AIN him was. identified as low as 350 degrees C by x-ray diffraction. The crystallization of the AlN film with (002) preferred orientation tended to improve with increased rf power and N-2 concentration within the experimental range. A densely pebble-like surface texture of the c-axis oriented ALN films with an average grain size of about 80 nm was observed by scanning electron microscopy (SEM). The film showed a high degree of alignment of the columnar structure as was indicated by the cross-sectional SEM photograph. The selected area diffraction pattern showed that the deposited films exhibited a polycrystalline microstructure.