A semiconductor device includes a substrate, a FinFET, and an insulating structure. The FinFET includes a fin, a gate electrode, and a gate dielectric layer. The fin is over the substrate. The gate electrode is over the fin. The gate dielectric layer is between the gate electrode and the fin. The insulating structure is over the substrate, adjacent the fin, and has a top surface lower than a top surface of the fin. The top surface of the insulating structure has opposite first and second edge portions and an intermediate portion between the first and second edge portions. The first edge portion of the top surface of the insulating structure is lower than the intermediate portion of the top surface of the insulating structure.