A method of fabricating a gate structure is provided. First, a sacrificial oxide layer is formed on a substrate. A nitridation treatment process is performed to redistribute the nitrogen atoms in the sacrificial layer and the substrate. Next, the sacrificial oxide layer is removed. A re-oxidation process is performed to produce an interface layer on the surface of the substrate. A high K (dielectric constant) gate dielectric layer, a barrier layer and a metal layer are sequentially formed on the substrate. The metal layer, the barrier layer, the high K gate dielectric layer and the interface layer are defined to form a stacked gate structure.