Semiconductor device are provided including a stacked structure having gate electrodes and interlayer insulating layers alternately stacked on a substrate; channel holes extending perpendicular to the substrate through the stacked structure and including channel regions therein; and horizontal parts at lower portions of the stacked structure and including areas in which the channel regions are horizontally elongated from the channel holes. The horizontal parts surround respective channel holes and are connected to each other between at a least portion of the channel holes.