A method for fabricating a fin-shaped semiconductor structure is provided, including: providing a semiconductor substrate with a semiconductor island and a dielectric layer formed thereover; forming a mask layer over the semiconductor island and the dielectric layer; forming an opening in the mask layer, exposing a top surface of the semiconductor island and portions of the dielectric layer adjacent to the semiconductor island; performing an etching process, simultaneously etching portions of the mask layer, and portions of the semiconductor island and the dielectric layer exposed by the opening; and removing the mask layer and the dielectric layer, leaving an etched semiconductor island with curved top surfaces and various thicknesses over the semiconductor substrate.