A nitride-based semiconductor element having excellent element characteristics is obtained by forming a nitride-based semiconductor layer having excellent crystallinity without performing a long etching process. This nitride-based semiconductor element and the method thereof includes forming a mask layer having a recess portion on a substantially flat upper surface of an underlayer to partially expose the upper surface of the underlayer. A nitride-based semiconductor layer is laterally grown on the exposed part of the underlayer and the mask layer while forming a void on the recess portion of the mask layer. Thus, the strain of the laterally grown nitride-based semiconductor layer is so relaxed that the crystallinity of the nitride-based semiconductor layer is improved.