A method for forming a semiconductor device structure is provided. The method includes forming a first fin structure over a substrate. The method includes forming a dielectric layer over the substrate and the first fin structure. The dielectric layer has a first trench exposing a first portion of the first fin structure. The method includes forming a first work function layer in the first trench. The method includes forming a first mask layer over the first work function layer in the first trench, wherein an upper portion of the first work function layer in the first trench is exposed by the first mask layer. The method includes removing the first work function layer exposed by the first mask layer. The method includes removing the first mask layer. The method includes forming a first gate electrode in the first trench.