A method of fabricating a gallium nitride (GaN) substrate provides a GaN thick film without causing bending and cracks which may occur in a growing process. To this end, a nitride embedding layer having a plurality of voids therein is embedded between a GaN layer and a base substrate. The method includes preparing a base substrate, growing, on the base substrate, the nitride embedding layer having a plurality of indium-rich parts at a first temperature, and growing a GaN layer on the nitride embedding layer at a second temperature higher than the first temperature so as to metallize the indium-rich part to form a plurality of voids in the nitride embedding layer.