Aluminum oxide (Al 2 O 3 ) thin films were deposited on Si (111) substrates by using RF magnetron sputtering of Al 2 O 3 target in Ar atmosphere. The synthesized films were annealed in the temperature range of 200 to 600°C in nitrogen (N 2 ) environment for 2 and 4 hours. Variations in these structural and surface properties of the films were investigated using X-ray diffraction (XRD) and atomic force microscope (AFM). XRD analysis reveals that the synthesized films are in polycrystalline form with preferential orientation along (111) plane. By increasing the annealing temperature, the crystallite size of films was found to increase, whereas the micro-strain and dislocation density were decreased. The decrease in micro-strain and dislocation density was ascribed to the reduction in the lattice strain. The surface roughness of the films was increased with the increase of the annealing temperature, which was attributed to the films' grains growth and also with the increase in RF sputtering power.