Abstract: A method for forming a semiconductor device structure is provided. The method includes forming a mandrel masking structure over a target layer. The method also includes patterning the mandrel masking structure to form mandrel lines parallel to each other, and forming spacer structures on sidewalls of the respective mandrel lines to define first openings. Each of the spacer structures includes a first spacer and a second spacer between the first spacer and the corresponding mandrel line. The method also includes removing the mandrel lines to define second openings, and etching the target layer through the first and second openings to form a target pattern therein.