A method for forming a semiconductor structure is provided. The method includes: forming first and second hard mask layers and a target layer on a substrate; patterning the second hard mask layer to form patterned second hard masks including a second wide mask and second narrow masks; and forming spacers on sidewalls of the second wide mask and the second narrow masks. Then, a photoresist layer is formed to cover the second wide mask and the spacers on the sidewalls of the second wide mask. The second narrow masks and the photoresist layer are removed. And, the first hard mask layer is etched with the spacers and the second wide mask together as a mask to form patterned first hard masks on the target layer, wherein the spacers define a first line width, and the second wide mask and the pair of spacers define a second line width.