A semiconductor device includes a substrate, at least one active region, at least one gate structure, and an insulating structure. The active region is present at least partially in the substrate. The gate structure is present on the active region. The gate structure has at least one end sidewall and a top surface intersecting to form a top interior angle. The top interior angle is an acute angle. The insulating structure is present adjacent to the end sidewall of the gate structure and on the substrate.