A semiconductor device includes a substrate, a lower stack structure on the substrate and including lower gate electrodes stacked apart from each other, an upper stack structure on the lower stack structure and including upper gate electrodes stacked apart from each other, a lower channel structure penetrating through the lower stack structure and including a lower channel layer, and a lower channel insulating layer on the lower channel layer the lower channel insulating layer surrounding a lower slit, and an upper channel structure penetrating through the upper stack structure and including an upper channel layer and an upper channel insulating layer on the upper channel layer, the upper channel insulating layer surrounding an upper slit. A width of the lower slit is greater than a width of the upper slit, and a thickness of the lower channel insulating layer is greater than a thickness of the upper channel insulating layer.