A method for forming a semiconductor device structure is provided. The method includes providing a substrate, a first nanostructure, and a second nanostructure. The method includes forming an isolation layer over the base. The method includes forming a gate dielectric layer over the first nanostructure, the second nanostructure, the fin, and the isolation layer. The method includes forming a gate electrode layer over the first part. The method includes forming a spacer layer. The method includes removing the second part of the gate dielectric layer and the first upper portion of the isolation layer to form a space between the fin and the spacer layer. The method includes forming a source/drain structure in the space and over the first nanostructure and the second nanostructure.