A method of manufacturing semiconductor device includes forming a plurality of sacrificial layers and a plurality of semiconductor layers repeatedly and alternately stacked on a substrate, partially removing the sacrificial layers, forming spacers in removed regions of the sacrificial layers, and replacing remaining portions of the sacrificial layers with a gate electrode. Each of the sacrificial layers includes first portions disposed adjacent to the plurality of semiconductor layers and a second portions disposed between the first portions. The second portion having a different composition from the first portions.