A method of fabricating a plurality of freestanding GaN wafers includes mounting a GaN substrate in a reactor, forming a GaN crystal growth layer on the GaN substrate through crystal growth, performing surface processing of the GaN crystal growth layer to form a GaN porous layer having a predetermined thickness on the GaN crystal growth layer, repeating the forming of the GaN crystal growth layer and the forming of the GaN porous layer a plurality of times to form a stack of alternating GaN crystal growth layers and GaN porous layers on the GaN substrate, and cooling the stack such that the GaN layers self-separate to form the freestanding GaN wafers. The entire process of forming a GaN porous layer and a thick GaN layer is performed in-situ in a single reactor. The method is very simplified compared to the prior art. In this way, the entire process is performed in one chamber, and in particular, GaN surface processing and growth proceed using an HVPE process gas such that costs are greatly reduced. In particular, since GaN is self-separated from the substrate through cooling, a GaN wafer of good quality without the occurrence of cracks can be produced on a large scale with a high yield.