A semiconductor device includes a substrate, a first stack structure on the substrate and includes a plurality of first gate electrodes, a second stack structure on the first stack structure and includes a plurality of second gate electrodes, a channel hole including a first lower channel hole that extends through a lower portion of the first stack structure, a first upper channel hole connected to the first lower channel hole, and a second channel hole connected to the first upper channel hole, and a channel structure in the channel hole. A side wall of the first lower channel hole has a first inclination relative to the first direction, a side wall of the first upper channel hole has a second inclination relative to the first direction, and a side wall of the second channel hole has a third inclination relative to the first direction.