Ultra-thin Al films were deposited with different deposition times on silicon wafer and copper grid by dc magnetron sputtering. The sputtering power of 200 watt and Ar flow rate of 20 sccm were used to prepare the films. The deposition times were 40, 120 and 240 second, respectively. The deposited Al films were, then, left in the air under the humidity of 60% for 20 days. The crystal structure of ultra-thin Al films deposited on silicon wafer and copper grid were investigated by glazing x-ray diffraction (GXRD) and transmission electron microscopy (TEM), respectively. The XRD results show that after the ultra thin Al films were exposed to the air, the Al was oxidized and the Al2O3 was formed at room temperature. In addition, Al deposited for 120 and 240 second can form polycrystalline of -Al2O3 with preferred orientations of (110) and (311) planes. The TEM images show that the particle size of -Al2O3 was about 8.5 nm for deposited time of 120 second. Moreover, the spectroscopic ellipsometry (SE) data and simulation model of Bruggemann effective medium approximation (BEMA) was used to determine the volume fraction of Al2O3.