A method for forming a patterned structure includes following steps. First lines elongated in a first direction and second lines elongated in a second direction in a layout pattern are decomposed into two masks. A first mask includes first line patterns and a first block pattern. A second mask includes second line patterns and a second block pattern. Two photolithography processes with the first mask and the second mask are performed for forming a patterned structure including first line structures and second line structures. Each first line structure is elongated in the first direction. The first line structures are defined by a region where the first line patterns and the second block pattern overlap with one another. Each second line structure is elongated in the second direction. The second line structures are defined by a region where the second line patterns and the first block pattern overlap with one another.