A semiconductor device includes a substrate, at least two gate spacers, and a gate stack. The substrate has at least one semiconductor fin. The gate spacers are disposed on the substrate. At least one of the gate spacers has a sidewall facing to another of the gate spacers. The gate stack is disposed between the gate spacers. The gate stack includes a high-κ dielectric layer and a gate electrode. The high-κ dielectric layer is disposed on the substrate and covers at least a portion of the semiconductor fin while leaving the sidewall of said at least one gate spacer uncovered. The gate electrode is disposed on the high-κ dielectric layer.