A method for forming a semiconductor device structure is provided. The method for forming a semiconductor device structure includes forming a fin structure over a substrate. The fin structure includes first semiconductor layers and second semiconductor layers alternately stacked. The method for forming the semiconductor device structure also includes removing the first semiconductor layers of the fin structure in a channel region thereby exposing the second semiconductor layers of the fin structure. The method for forming the semiconductor device structure also includes forming a dielectric material surrounding the second semiconductor layers, and treating a first portion of the dielectric material. The method for forming the semiconductor device structure also includes etching the first portion of the dielectric material to form gaps, and filling the gaps with a gate stack.