A semiconductor device includes a peripheral circuit structure including a plurality of circuits, and a cell array structure overlapping the peripheral circuit structure in a vertical direction. The cell array structure includes a common source line, a stack structure including a plurality of gate layers and a plurality of interlayer insulating layers which are alternately stacked on the common source line, and a plurality of channel structures in channel holes penetrating a memory cell area of the stack structure and connected to the common source line. Each of the plurality of channel structures includes a channel layer including an upper channel layer and a lower channel layer each having a single-crystal structure, and a crystal orientation of the upper channel layer is different from a crystal orientation of the lower channel layer.