A semiconductor device includes a substrate, at least one gate, and an insulating structure. The substrate includes at least one semiconductor fin. The gate is disposed on the semiconductor fin. The gate has at least one end sidewall. The insulating structure is disposed adjacent to the gate. The insulating structure has a sidewall facing the gate, and the end sidewall of the gate is in contact with a portion of the sidewall of the insulating structure while leaves another portion of the sidewall of the insulating structure uncovered.