A method of preparing a nitride semiconductor capable of forming a nitride-based semiconductor layer having a small number of dislocations as well as a small number of crystal defects resulting from desorption with excellent crystallinity on the upper surface of a substrate through a small number of growth steps is proposed. The method of preparing a nitride-based semiconductor comprises steps of forming a mask layer on the upper surface of a substrate to partially expose the upper surface of the substrate, forming a buffer layer on the exposed part of the upper surface of the substrate and the upper surface of the mask layer and thereafter growing a nitride-based semiconductor layer. Thus, the outermost growth surface of the nitride-based semiconductor layer laterally grown on the mask layer does not come into contact with the mask layer. Therefore, desorption hardly takes place from the outermost growth surface of the nitride-based semiconductor layer, whereby a nitride-based semiconductor layer having a small number of defects is formed. Further, the mask layer is directly formed on the substrate, whereby the number of growth steps for the nitride-based semiconductor layer is reduced.