A semiconductor device includes: a first gate stack including a plurality of first gate electrodes; a second gate stack arranged on the first gate stack and including a plurality of second gate electrodes; and a plurality of channel structures arranged in a plurality of channel holes penetrating the first gate stack and the second gate stack. Each of the channel holes includes a first channel hole portion penetrating the first gate stack and a second channel hole portion penetrating the second gate stack, and a ratio of a second width in the second direction to a first width in the first direction of an upper end of the first channel hole portion is less than a ratio of a fourth width in the second direction to a third width in the first direction of an upper end of the second channel hole portion.