A semiconductor device includes a gate structure on a substrate, the gate structure including insulating layers and gate electrodes, which are alternately stacked, a channel structure extending through the gate structure, and a source conductive pattern between the substrate and the gate structure. The source conductive pattern includes a lower source conductive pattern and an upper source conductive pattern on the lower source conductive pattern. The channel structure includes an insulating pattern extending through the source conductive pattern, a data storage pattern, and a channel pattern between the insulating pattern and the data storage pattern. A lower surface of the channel pattern is at a level higher than an upper surface of the upper source conductive pattern, but lower than a lower surface of a lowermost one of the gate electrodes in a cross-sectional view of the semiconductor device with the substrate providing a base reference level.