A semiconductor device, includes: gate electrodes spaced apart from each other and on a substrate; channel structures penetrating the gate electrodes, each of channel structures including a channel layer, a gate dielectric layer between the channel layer and the gate electrodes, a channel insulating layer filling between the channel layers, a channel pad on the channel insulating layer; and separation regions penetrating the gate electrodes, and spaced apart from each other, wherein the gate dielectric layer extends upwardly, further than the channel layer upwardly such that a portion of an inner side surface of the gate dielectric layer contacts the channel pad, the channel pad includes a lower pad on an upper end of the channel layer and the inner side surface of the gate dielectric layer, and having a first recess between the inner side surfaces of the gate dielectric layer; and an upper pad having a first portion in the first recess and a second portion extending from the first portion in a direction, parallel to an upper surface of the substrate on the first portion.