A three-dimensional semiconductor device including a conductive layer disposed on a substrate and including a first conductivity-type impurity; an insulating base layer disposed on the conductive layer; a stack structure including a lower insulating film disposed on the insulating base layer, and a plurality of gate electrodes and a plurality of mold insulating layers alternately stacked on the lower insulating film, wherein the insulating base layer includes a high dielectric material; a vertical structure including a vertical channel layer penetrating through the stack structure and a vertical insulating layer disposed between the vertical channel layer and the plurality of gate electrodes, the vertical structure having an extended area extending in a width direction in the insulating base layer; and an isolation structure penetrating through the stack structure, the insulating base layer and the conductive layer, and extending in a direction parallel to an upper surface of the substrate, wherein the conductive layer has an extension portion extending along a surface of the vertical channel layer in the extended area of the vertical structure.