Copper Indium disulfide, CuInS2 or CIS films were prepared by thermal evaporation technique. Product films were prepared on glass substrates and heat-treated at various temperatures at fixed time. Structural and optical properties were investigated by using energy dispersive spectroscopy (EDS),X-Ray diffraction (XRD), Scanning electronmicroscope (SEM) and optical reflection and transmission measurements. The optical band gap increases within range 1.47 eV for the as prepared films to 1.61 eV for the annealed one. The annealing was performed within temperature range 523 up to 723 K. The increasing of the optical gap was attributed to both nano structure property and the enhancements of grain size. The obtained results were discussed in the light of current theoretical ideas.