Aluminum-doped zinc oxide thin films were deposited on glass substrates by the sol-gel technique. The effect of the Al concentration in the starting solution on the structural and optical properties of ZnO : Al thin films were studied. The molar ratio of the dopant (aluminum nitrate) in the solution [Al/Zn] was varied between 1% and 3%.The deposition of the layers was realized performed at room temperature by the dip-coating technique. The obtained films were inserted to a furnace and annealed at 500°C for 60 min. The X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), and optical transmittance were used to characterize the elaborated samples. XRD results show that the films have hexagonal structure and grow preferentially along the crystallographic direction (002). The average crystallite size estimated by the Scherrer formula is about 5 nm. FT-IR spectrometry showed all types of molecular vibrations in the films and confirmed the formation of a ZnO semiconductor in the samples. The refractive index and thickness of layers were determined from the optical transmittance spectra. We observed that an increase in the Al content in the solution leads to an increase in the films thickness and a decrease in their refractive index.