A semiconductor device includes a semiconductor fin, a gate structure, a source epitaxy structure and a drain epitaxy structure. The semiconductor fin extends along a first direction above a substrate. The gate structure extends across the semiconductor fin along a second direction different from the first direction. The gate structure includes a gate dielectric layer wrapping around the semiconductor fin and a chlorine-containing N-work function metal layer wrapping around the gate dielectric layer. The source epitaxy structure and the drain epitaxy structure are on opposite sides of the gate structure, respectively.