A method for manufacturing a semiconductor device having a metal gate includes forming a filling layer and a high-K gate dielectric layer in the first recess between a pair of spacers, wherein the high-K gate dielectric layer and the filling layer are stacked in the first recess sequentially, and an exposed top surface of the high-K gate dielectric layer and a top surface of the filling layer are lower than a top surface of each spacer; and removing a part of each spacer and widening the first recess on the top surface of the filling layer to form a second recess, wherein a width of the second recess is larger than a width of the first recess.