A read-only memory (ROM) structure is provided. The ROM device structure includes a first gate structure formed over a substrate, and the first gate structure includes a first work function layer with a first thickness. The ROM device structure includes an isolation structure formed over the substrate, and the isolation structure is adjacent to the first gate structure. The isolation structure includes a second work function layer with a second thickness, and the second thickness is larger than or smaller than the first thickness. The ROM device structure also includes a first contact structure formed over the substrate, and the first contact structure is between the first gate structure and the isolation structure.