A semiconductor device includes a substrate, a stack structure including interlayer insulating layers and gate electrodes alternately and repeatedly stacked on the substrate in a first direction perpendicular, a channel structure that penetrates the stack structure, a contact plug disposed on the channel structure, and a bit line on the contact plug. The channel structure includes a core pattern, a pad structure on the core pattern, and a channel layer on a side surface of the core pattern and a side surface of the pad structure. The pad structure includes a pad pattern, a first pad layer, and a second pad layer, the first pad layer that is between the channel layer and the pad pattern, and the second pad layer including a first portion between the channel layer and the first pad layer, and a second portion between the first pad layer and the core pattern.