A semiconductor device includes a substrate having a cell region and a connection region, a first stack structure with a plurality of first gate layers and a plurality of first interlayer insulating layers, and a second stack structure with a plurality of second gate layers and a plurality of second interlayer insulating layers . Each of the first gate layers includes a central portion in the cell region of the substrate and an end portion in the connection region of the substrate. Each of the second gate layers includes a central portion in the cell region of the substrate and an end portion in the connection region of the substrate. A thickness difference between the end and central portions of each first gate layer is different from a thickness difference between the end and central portions of each second gate layer.