The optical properties of Al₂O₃ films grown by atomic layer deposition were investigated. Trimethylaluminium (TMA) and water vapour (H2O) were used as the chemical precursors to deposit Al₂O₃ films on glass substrates at temperature of 250 ℃ and 300 ℃ respectively. Characterization of the films such as optical properties,surface morphological image and microstructure were performed by using spectrophotometer,X-ray diffraction (XRD),X-ray photoelectron spectroscopy (XPS),scanning electron microscope (SEM) and atomic force microscopy (AFM).The results show that both the as-deposited and annealed A12O3 films are amorphous. The surface roughness is as low as 1.2 nm while the packing densities are larger than 0.97. Al₂O₃ films by this technique exhibit good optical properties with low absorption in the spectral region from mid-ultraviolet to near-infrared. This study indicates that Al₂O₃ film deposited by ALD is applicable for optical coatings of the materials with midde and low refractive index.