AIN films grown on sapphire by MOCVD with different V/IAR ratios were investigated by XRD, TEM and AFM. The AIN films show single crystalline character as well as columnar structure. The growth of AIN has three zones: (1) high-density nucleation layer (2) fine columnar growth and (3) grain merging and lateral growth. The films grown at intermediate V/III ratio have the maximum value of  FWHM. When the V/HI ratio increases, the thickness of the nucleation zone and the film misorientation increase, but when the ratio is increased further, the nucleation zone decreases and the AIN film has a more highly oriented growth. These results suggest that the AIN films with optimum crystalline quality can be obtained by varying the V/III ratio during growth.