The present disclosure provides a method of fabricating a semiconductor device. The method includes providing a semiconductor substrate having a first region and a second region, forming a high-k dielectric layer over the semiconductor substrate, forming a capping layer over the high-k dielectric layer in the first region, forming a first metal layer over capping layer in the first region and over the high-k dielectric in the second region, thereafter, forming a first gate stack in the first region and a second gate stack in the second region, protecting the first metal layer in the first gate stack while performing a treatment process on the first metal layer in the second gate stack, and forming a second metal layer over the first metal layer in the first gate stack and over the treated first metal layer in the second gate stack.