A memory device includes a plurality of gate electrode layers stacked on a substrate, a plurality of channel layers penetrating the plurality of gate electrode layers, a gate insulating layer between the plurality of gate electrode layers and the plurality of channel layers, and a common source line on the substrate adjacent to the gate electrode layers. The common source line includes a first part and a second part that are alternately arranged in a first direction and have different heights in a direction vertical to a top surface of the substrate. The gate insulating layer includes a plurality of vertical parts and a horizontal part. The plurality of vertical parts surrounds corresponding ones of the plurality of channel layers. The horizontal part extends parallel to a top surface of the substrate.