A method for manufacturing a structure having an air gap includes following steps. A plurality of patterns is formed in a pattern region of a substrate. A sacrificial layer is formed on the substrate, and a top surface of the sacrificial layer is lower than a top surface of the patterns to expose a plurality of upper portions of the patterns. A hard mask layer is formed to cover the sacrificial layer and the upper portions of the patterns. An etching-back process is performed to the hard mask layer to expose the sacrificial layer outside the pattern region, and the hard mask layer remaining inside the pattern region seals the opening between the upper portions of the patterns. The sacrificial layer is removed to form an air gap between the two adjacent patterns.