A FinFET device structure and method for forming the same are provided. The method includes forming a fin structure over a substrate and forming a dummy gate electrode over a middle portion of the fin structure. The method also includes forming a spacer layer on the dummy gate electrode and on the fin structure and performing a plasma doping process on the dummy gate electrode and on the spacer layer. The method further includes performing an annealing process, wherein the annealing process is performed by using a gas comprising oxygen, such that a doped region is formed in a portion of the fin structure, and the spacer layer is doped with oxygen after the annealing process.