A method for forming a semiconductor device structure is provided. The method includes forming a dummy gate stack over a semiconductor substrate and forming spacer elements over sidewalls of the dummy gate stack. The method also includes removing the dummy gate stack to form a recess between the spacer elements and partially removing the spacer elements such that an upper portion of the recess becomes wider. The method further includes forming a metal gate stack in the recess and forming a protection element in the recess to cover the metal gate stack.