A series of GaN layers was grown on sapphire (0001) substrates under various growth conditions by means of the molecular beam epitaxy(MBE) method, the optical characteristics and surface morphologies of the samples were studied. The results show that the line width of the GaN emission gradually decreases and the peak shifts under the Ga-rich condition by increasing the Ga-flux on keeping all other growth conditions unchanged. It has been also found that the resulted morphology is directly related to the Ga-flux.