Abstract: The growth of GaN on sapphire by RF plasma assisted molecular beam epitaxy （RF plasma MBE） is investigated with the object of optimizing the material quality. The small FWHM of x-ray diffraction rocking curve and 77K photoluminescence （335 arcsec and 22meV, respectively） show that the epitaxial GaN has good quality. Using mosaic model, the dislocation density in the GaN is estimated to be 7.3×10⁸cm^-2. The optical properties of n-GaN are investigated for Si doping concentration ranging from 10¹⁷ to 10¹⁹ cm^-2. The intensity of the near -band-gap transition is found to increase monotonically as the doping concentration is increased.