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The microstructure, optical absorption properties and luminescence function of GaN thin films grown on the sapphire substrate with the buffer layer of GaN by MOCVD have been studied by means of XRD, TEM, infrared transmission spectrum, and photoluminescence. The XRD results show that the crystal of GaN is hexagonal wurtzite structure. The thin films have perferred orientation in c axis with very high quality. The TEM images of cross-sectional specimen show that the thicknesses of every layer in the superlattices are uniform with the average period of 13.3 nm, but there are high-density dislocations in the superlattices region. With the related optical experimental data, it had been found that the optical absorption edge was at about 370 nm. The theory calculation indicates that the five samples are direct transition semiconductor with band-gaps of about 3.4 eV. The refractive indexes of the samples increase with the photon energy enhance and decrease with the wavelength increasing. The results show that the extinction coefficients have the minimum at 370 nm. Photoluminescence test results show that the superlattice has preferable luminescence property and a yellow luminescence is found.