A method for fabricating semiconductor device includes the steps of first forming a gate dielectric layer on a substrate; forming a gate material layer on the gate dielectric layer, and removing part of the gate material layer and part of the gate dielectric layer to form a gate electrode, in which a top surface of the gate dielectric layer adjacent to two sides of the gate electrode is lower than a top surface of the gate dielectric layer between the gate electrode and the substrate. Next, a first mask layer is formed on the gate dielectric layer and the gate electrode, part of the first mask layer and part of the gate dielectric layer are removed to form a first spacer, a second mask layer is formed on the substrate and the gate electrode, and part of the second mask layer is removed to form a second spacer.