GaN nanowires (NWs) have been grown on Si (111) substrates byt plasma-assisted moleucular bearn epitaxy. The Morphology and optical properties of the NWs are influenced by te growth parameters as investigated by the scanning electron microscope. The nucleation process of GaN-NWs is explained in terms of nucleation density and wire evolution with time. The wire length in the nucleation stage shows a linear time dependence. The wire density increases rapidly with time and then it saturates. We explain GaN-NWs growth by making use of the diffusion-induced (DI) mechanism that explains the dependence of the length on wire diameter for a deposition time longer than the nucleation stage.