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 and produce a concentration profile such that the concentration of nitrogen progressively increases and then decreases toward the substrate with the maximum concentration of nitrogen in the sacrificial oxide layer. 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.