A manufacturing process of an etch stop layer is provided. The manufacturing process includes steps of providing a substrate; forming a gate stack structure over the substrate, wherein the gate stack structure at least comprises a dummy polysilicon layer and a barrier layer; removing the dummy polysilicon layer to define a trench and expose a surface of the barrier layer; forming a repair layer on the surface of the barrier layer and an inner wall of the trench; and forming an etch stop layer on the repair layer. In addition, a manufacturing process of the gate stack structure with the etch stop layer further includes of forming an N-type work function metal layer on the etch stop layer within the trench, and forming a gate layer on the N-type work function metal layer within the trench.