A method of manufacturing a semiconductor device includes forming a plurality of strings spaced a first distance from each other, each string including first preliminary gate structures spaced a second distance, smaller than the first distance, between second preliminary gate structures, forming a first insulation layer to cover the first and second preliminary gate structures, forming an insulation layer structure to fill a space between the strings, forming a sacrificial layer pattern to partially fill spaces between first and second preliminary gate structures, removing a portion of the first insulation layer not covered by the sacrificial layer pattern to form a first insulation layer pattern, reacting portions of the first and second preliminary gate structures not covered by the first insulation layer pattern with a conductive layer to form gate structures, and forming a capping layer on the gate structures to form air gaps between the gate structures.