A method of fabricating a freestanding gallium nitride (GaN) substrate includes: preparing a GaN substrate within a reactor; supplying HCl and NH3 gases into the reactor to treat the surface of the GaN substrate and forming a porous GaN layer; forming a GaN crystal growth layer on the porous GaN layer; and cooling the GaN substrate on which the GaN crystal growth layer has been formed and separating the GaN crystal growth layer from the substrate. According to the fabrication method, the entire process including forming a porous GaN layer and a thick GaN layer is performed in-situ within a single reactor. The method is significantly simplified compared to a conventional fabrication method. The fabrication method enables the entire process to be performed in one chamber while allowing GaN surface treatment and growth to be performed using HVPE process gases, thus resulting in a significant reduction in manufacturing costs. The fabrication method also allows self-separation of thick GaN without cracking, thus achieving a short process time and a high manufacturing yield.