Ir(acac)3 and O2 are used as chemical precursors to deposit iridium thin films on Si and quartz substrates by atomic layer deposition(ALD) at the temperature of 340 ℃. Characteristics of the films such as optical properties, microstructure and surface morphological image are investigated by reflectance spectrometer, X-ray diffraction(XRD), X-ray photoelectron spectroscopy(XPS), scanning electron microscope(SEM) and atomic force microscopy (AFM). The results show that the films have polycrystalline morphology of nanoparticles and smooth surfaces, low impurity content. The thin films exhibits good optical properties in the ultraviolet spectral region, and it can be well used for Ir grating and other optical devices.