Abstract: A semiconductor device includes a memory cell structure on a substrate, and a dummy structure on a side of the memory cell structure. The memory cell structure includes a memory stack structure including interlayer insulating layers and gate electrodes alternately stacked on the substrate, channel structures penetrating through the memory stack structure and contacting the substrate, and first separation structures penetrating through the memory stack structure and extending in the first direction to separate the gate electrodes from each other in a second direction. The dummy structure includes dummy stack structures spaced apart from the memory stack structure and including first insulating layers and dummy gate electrodes alternately stacked, dummy channel structures penetrating through the dummy stack structures, and second separation structures penetrating through the dummy stack structures and extending in the second direction to separate the dummy gate electrodes from each other in the first direction.