A semiconductor device includes a substrate comprising a semiconductor fin, a gate structure over the semiconductor fin, and source/drain structures over the semiconductor fin and on opposite sides of the gate structure. The gate stack comprises a high-k dielectric layer; a first work function metal layer over the high-k dielectric layer; an oxide of the first work function metal layer over the first work function metal layer; and a second work function metal layer over the oxide of the first work function metal layer, in which the first and second work function metal layers have different compositions; and a gate electrode over the second work function metal layer.