A semiconductor device includes lower gate electrodes on a substrate in a first direction substantially perpendicular to a top surface of the substrate, upper gate electrodes on the lower gate electrodes in the first direction, and channel structures extending through the lower and upper gate electrodes in the first direction. Each channel structure includes a lower channel structure, an upper channel structure, and a landing pad interconnecting the lower and upper channel structures. The first channel structure includes a first landing pad having a horizontal width substantially greater than that of the lower channel structure of the first channel structure at a first vertical level. The second channel structure located closest to the first channel structure includes a second landing pad having a horizontal width substantially greater than that of the lower channel structure of the second channel structure at a second vertical level lower than the first vertical level.