A method includes forming a plurality of dummy gate structures on a substrate, each dummy gate structure including a dummy gate electrode and a dummy gate mask, forming a first insulation layer on the substrate and the dummy gate structures to fill a first space between the dummy gate structures, planarizing upper portions of the first insulation layer and the dummy gate structures, removing the remaining first insulation layer to expose a portion of the substrate, forming an etch stop layer on the remaining dummy gate structures and the exposed portion of the substrate, forming a second insulation layer on the etch stop layer to fill a second space between the dummy gate structures, planarizing upper portions of the second insulation layer and the etch stop layer to expose the dummy gate electrodes, removing the exposed dummy gate electrodes to form trenches, and forming metal gate electrodes in the trenches.