An optical semiconductor element and a manufacturing method thereof that can improve the light extraction efficiency with maintaining the yield. The manufacturing method includes forming a plurality of recesses arranged at equal intervals along a crystal axis of a semiconductor film in a surface of the semiconductor film; and performing an etching process on the surface of the semiconductor film, thereby forming a plurality of protrusions arranged according to the arrangement form of the plurality of recesses and deriving from the crystal structure of the semiconductor film in the surface of the semiconductor film.