In a method for fabricating a nitride-based compound layer, first, a GaN substrate is prepared. A mask layer with a predetermined pattern is formed on the GaN substrate to expose a partial area of the GaN substrate. Then a buffer layer is formed on the partially exposed GaN substrate. The buffer layer is made of a material having a 10% or less lattice mismatch with GaN. Thereafter, the nitride-based compound is grown laterally from a top surface of the buffer layer toward a top surface of the mask layer and the nitride-based compound layer is vertically grown to a predetermined thickness. Also, the mask layer and the buffer layer are removed via wet-etching to separate the nitride-based compound layer from the GaN substrate.