A semiconductor device according to an example embodiment includes a substrate extending in first and second directions intersecting with each other; nanowires on the substrate and spaced apart from each other in the second direction; gate electrodes extending in the first direction and spaced apart from each other in the second direction, and surrounding the nanowires to be superimposed vertically with the nanowires; external spacers on the substrate and covering sidewalls of the gate electrodes on the nanowires; and an isolation layer between the gate electrodes and extending in the first direction, wherein an upper surface of the isolation layer is flush with upper surfaces of the gate electrodes.