Unintentionally doped wurtzite GaN grown on (0001) sapphire was studied by spectroscopic ellipsometry(SE) in the wavelength range of 400～1200nm. Both the thickness and dispersion of the refractive index of GaN, i.e. n₂(λ)=2.26₂+330.1₂/((λ/nm)₂-265.7₂), were obtained by fitting the experimental data. With this formula, the reflection spectrum of GaN in the UV-VIS range was studied and the thickness of GaN was calculated. The two values of the thickness obtained by these two different methods are in good agreement and the deviation is found to be 0.68%.