The present invention relates to a method of forming a nitride semiconductor substrate. This method includes steps of providing a substrate and then forming an epitaxy layer on the substrate. A patterned mask layer is formed on the epitaxy layer, wherein the patterned mask layer exposes a portion of the epitaxy layer. Next, an oxidation process is performed to oxidize the exposed epitaxy layer so as to form a plurality of dislocation blocking structures. The patterned mask layer is then removed. Further, a nitride semiconductor layer is formed on the epitaxy layer having the dislocation blocking structures.