A semiconductor device includes a substrate, a plurality of gate electrodes extending in a first direction parallel to an upper surface of a substrate on the substrate, and alternately arranged with an interlayer insulating layer in a second direction perpendicular to the upper surface of the substrate, a vertical channel layer on a sidewall of a vertical channel hole extending in the second direction by penetrating through the plurality of gate electrodes and the interlayer insulating layer, and connected to the upper surface of the substrate, and a first gap-fill insulating layer formed in the vertical channel hole and including an outer wall contacting the vertical channel layer and an inner wall opposite the outer wall, wherein a part of the inner wall forms a striation extending in the second direction.