Nanostructured CuO thin films were produced from alkaline baths containing saccharin as additive. X-ray diffraction analysis was used to determine the phase, crystallization and average crystallite size of the films. From the XRD analysis it was understood that the films have polycrystalline structure with monoclinic phases. The average crystallite size was also studied by the scanning electron microscopy. SEM images showed that the films were consisted of plate-like nanostructures with the thicknesses of ∼25nm. The effect of saccharin concentration on the transmission and optical band gap were also studied by UV–vis spectrophotometry. From the UV–vis spectra of the films it was seen that both the optical band gap and transmission properties of the films change with increasing saccharin concentration in the growth bath.