The present disclosure provides a method of fabricating a semiconductor device. A high-k dielectric layer is formed over a substrate. A first capping layer is formed over a portion of the high-k dielectric layer. A second capping layer is formed over the first capping layer and the high-k dielectric layer. A dummy gate electrode layer is formed over the second capping layer. The dummy gate electrode layer, the second capping layer, the first capping layer, and the high-k dielectric layer are patterned to form an NMOS gate and a PMOS gate. The NMOS gate includes the first capping layer, and the PMOS gate is free of the first capping layer. The dummy gate electrode layer of the PMOS gate is removed, thereby exposing the second capping layer of the PMOS gate. The second capping layer of the PMOS gate is transformed into a third capping layer.