A semiconductor device includes gate electrodes and interlayer insulating layers alternately stacked on a substrate, a channel layer penetrating through the gate electrodes and the interlayer insulating layers, and a gate dielectric layer disposed on an external surface of the channel layer between the gate electrodes and the channel layer. In addition, the channel layer includes a first region extended in a direction perpendicular to a top surface of the substrate and a second region connected to the first region in a lower portion of the first region and including a plane inclined with respect to the top surface of the substrate.