The microstructure, optical absorption properties and luminescence function of GaN thin films grown on the sapphire substrate with the buffer layer of GaN by metal-organic chemical vapor deposition (MOCVD) have been studied by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), infrared transmission spectrum, ultraviolet-visible absorption spectrum and photoluminescence. XRD results show that the crystal structure of GaN is hexagonal wurtzite structure. The thin films have preferred orientation in c axis with very high quality. TEM images of the cross-sectional specimen show that the thickness of every layer in the superlattice are uniform with the average period of 13.3 nm, but there are high-density dislocations in the superlattices region. From related optical experimental data, it is found that optical absorption edge is at about 370 nm. The theory calculation indicates that the five samples are direct transition semiconductor and band-gaps are about 3.4 eV. The refractive indexes of the samples increase with photon energy enhancing and decrease with the wavelength increasing. The results show that the extinction coefficients reach the lowest point at 370 nm. Photoluminescence test results show that superlattice has preferable luminescence property. In addition, the yellow luminescence is found in all samples.