A method for forming a semiconductor structure is provided. The method includes forming a first fin structure over a first region of a substrate and forming a second fin structure over a second region of a substrate, forming a first gate dielectric layer around the first fin structure and forming a second gate dielectric layer around the second fin structure, forming a barrier layer over the first gate dielectric layer, treating the substrate with a first fluorine-containing gas, forming a work function layer over the second gate dielectric layer after treating the substrate with the first fluorine-containing gas, and treating the substrate with a second fluorine-containing gas after forming the work function layer over the second gate dielectric layer.