This work reports the preparation of Al-Ti based oxide thin films and their optical properties. Although the transmittance of a TiO₂/Al₂O₃ bilayer structure was as high as 90% at wavelengths of 600 nm or larger, the reflectance of the bilayer reached its minimum at wavelengths of around 360 nm. The transmittance of an 89-nm-thick TiO₂ thin film rapidly increased and then decreased at a critical wavelength because of destructive interference. The wavelength corresponding to the reflectance minimum increased after an increase in TiO₂ film thickness. The smooth surface morphology of the AlTiO thin film was retained up to a film thickness of 65 nm, and the transmittance of the film was inversely proportional to film thickness, in accordance with the general tendency for optical films. The reflectance of the AlTiO film at visible light wavelengths was lower than that of the TiO₂ film, which implies that the AlTiO film is suitable for applications as an optical thin film layer in semitransparent solar cells.