A semiconductor device includes a substrate including a first plate portion and a second plate portion, a stack structure including interlayer insulating layers and gate electrodes alternately stacked on the substrate, a first block separation structure on the first plate portion and a second block separation structure on the first plate portion. Each of the first and second block separation structures includes first separation regions, a cell array separation structure including a second separation region connected to the first separation regions and channel structures penetrating the stack structure, wherein the stack structure includes first stack structures separated by the first separation regions of the first block separation structure and extending in the first direction, second stack structures separated by the first separation regions of the second block separation structure, and at least one third stack structure separated from the first and second stack structures by the cell array separation structure.