A method for forming a dual damascene opening includes the following steps. Firstly, a first hard mask layer with a trench pattern is formed over a material layer. Then, a dielectric layer is formed over the first hard mask layer and filled into an opening of the trench pattern. Then, a second hard mask layer with a via opening pattern is formed over the first hard mask layer and the dielectric layer. Then, a first etching process is performed, so that a via opening is at least formed in the dielectric layer. After the second hard mask layer is removed, a second etching process is performed. Consequently, a trench opening is formed in the material layer and the via opening is further extended into the material layer, wherein the via opening is located within the trench opening.