Provided are a gate structure and a method of forming the same. The gate structure includes a gate dielectric layer, a metal layer, and a cluster layer. The metal layer is disposed over the gate dielectric layer. The cluster layer is sandwiched between the metal layer and the gate dielectric layer, wherein the cluster layer at least includes an amorphous silicon layer, an amorphous carbon layer, or an amorphous germanium layer. In addition, a semiconductor device including the gate structure is provided.