Disclosed is a method for fabricating a GaN semiconductor epitaxial layer. The method includes the steps of: (a) providing a substrate within a reaction furnace; (b) setting a temperature range of the substrate to be 200° C.˜1,300° C.; (C) supplying a Ga metallic source on the substrate; (d) changing the supplied Ga metallic source on the substrate, to Ga metal islands; (e) supplying a nitrogenous source to the Ga metal islands after suspending supply of the Ga metallic source; (f) forming GaN islands by reacting the Ga metal islands with the nitrogenous source; and (g) growing a GaN epitaxial layer by basing the GaN islands as a seed.