A semiconductor device includes a substrate, a fin structure on the substrate, a gate structure on the fin structure, a gate spacer on at least on side surface of the gate structure, and a source/drain structure on the fin structure, wherein a topmost portion of a bottom surface of the gate spacer is lower than a topmost portion of a top surface of the fin structure, and a topmost portion of a top surface of the source/drain structure is lower than the topmost portion of the top surface of the fin structure.