High quality GaN films with low dislocation density have been grown on sapphire substrate which is pretreated by a new and simple method in order to overcome those problems existing in the conventional ELO technique. Etch pits on sapphire substrate surface are formed after surface treating. GaN films have been grown by LP-MOCVD on the sapphire substrate, which a half of it is treated by chemical etch. Both the thickness and dispersion of the refractive index of GaN films are obtained by spectroscopic ellipsometry. With the dispersion of the refractive index, the transmission spectrum of GaN is studied and the thickness of GaN epilayer is calculated. The two values of the thickness obtained by these two different methods are in good agreement. The epilayer grown on the surface treated sapphire substrate exhibits superior optical properties and crystal properties, in which the yellow luminescence is nearly invisible in the photoluminescence spectrum, the higher transmission ratio and the greater modulation depth can be shown in the transmission spectrum.