A ZnO: Al (10%) thin film was prepared on GaAs(100) substrate by using a pulsed filtered cathodic vacuum arc deposition (PFCVAD) system. The ZnO: Al thin film was thermally annealed for 1 h at 2 different temperatures in air. The film structure was investigated as a function of annealing temperature by X-ray diffraction (XRD). ZnO: Al film annealed at 500 degrees C by PFCVAD method resulted in an amorphous film. However, after annealing ZnO: Al film at 600 degrees C, it showed (100) and (002) peaks at around 32 degrees and 34 degrees, respectively. The chemical state of ZnO: Al (AZO) film on GaAs substrate was investigated by using X-ray photoelectron spectroscopy (XPS). The dependence of optical properties on annealing was investigated by using a UV-VIS-NIR spectrophotometer. We also obtained the energy gap of ZnO: Al thin film by diffused reflectance spectra using the Kubelka-Munk function.