The present disclosure provides a method of fabricating a semiconductor device. The method includes providing a semiconductor substrate with a first region and a second region, forming a high-k dielectric layer over the semiconductor substrate, forming a metal layer over the high-k dielectric layer, the metal layer having a first work function, protecting the metal layer in the first region, treating the metal layer in the second region with a de-coupled plasma that includes carbon and nitrogen, and forming a first gate structure in the first region and a second gate structure in the second region. The first gate structure includes the high-k dielectric layer and the untreated metal layer. The second gate structure includes the high-k dielectric layer and the treated metal layer. The invention provides a simple single metal layer with economic efficiency, suitable for the prior CMOS technical flow path, therefore it can be easily integrated with the prior manufacturing facility and device technology.