Aluminium nitride thin films have been fabricated on silicon wafers by reactive r.f. magnetron sputtering in mixed Ar-N-2 discharge with variation of negative bias voltage. The effect of negative bias voltage on the 2 microstructures of AlN thin films have been investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), stress measurement, Auger electron spectroscopy (AES), etc. While the negative bias voltage was varied in the range 0 to -45 V, highly c-axis oriented film can be fabricated at -30 V, and the grain size and compressive stress increase with the negative bias voltage. From the plasma analysis, the dominant positive chemical species is identified as N-2(+) ions. The above results can be understood considering that the kinetic energy transfer and flux of N-2(+) ions increases with increasing negative bias voltage.