An optical semiconductor element includes a single crystal AlN substrate; an n-type semiconductor layer having an AlGaN layer, the AlGaN layer being grown on the AlN substrate and being pseudomorphic with the AlN substrate, an Al composition of the AlGaN layer being reduced with an increase in distance from the AlN substrate; an active layer grown on the n-type semiconductor layer and having a multiple quantum well structure which includes a plurality of well layers and barrier layers; and a p-type semiconductor layer which is grown on the active layer. The single crystal AlN substrate has a dislocation density being 106 cm−2 or less.