A semiconductor device includes a substrate having first and second regions, gate electrodes stacked in a first direction perpendicular to the substrate in the first region, and extending by different lengths in a second direction perpendicular to the first direction in the second region, first separation regions in the first and second regions through the gate electrodes, extending in the second direction, and spaced apart from each other in a third direction perpendicular to the first and second directions, second separation regions between the first separation regions through the gate electrodes and extending in the second direction, portions of the second separation regions being spaced apart from each other in the second direction in the second region, and an insulation region extending in the third direction to separate at least one of the gate electrodes into portions adjacent to each other in the second direction.