Abstract: A method for fabricating an optical semiconductor device includes the steps of forming at least two stripes of dielectric parallel to each other with a predetermined interval on a semiconductor substrate, growing a crystal selectively between the two stripes, and forming a multi-layer structure which is required to have a width determined by the crystal grown between the two stripes. In such a method, the width of the multi-layer structure including an active layer or a waveguide is controlled precisely, because there is no step of etching a semiconductor layer, so that the characteristics of the device may improve and the yield may increase.