There are provided a memory device and a manufacturing method thereof. A method of manufacturing a memory device may include forming, on a substrate, a conductive layer, a sacrificial layer, and a stack structure. The method may include forming a plurality of vertical holes by etching a portion of the stack structure. The method may include forming a memory layer and a channel layer along internal surfaces of the vertical holes. The method may include forming a slit trench exposing a portion of the sacrificial layer therethrough by etching a portion of the stack structure between the vertical holes. The method may include exposing a portion of the channel layer and the first conductive layer through a lower portion of the stack structure by removing portions of the sacrificial layer and the memory layer. The method may include forming another conductive layer along surfaces of the exposed portion of the channel layer and the first conductive layer. The method may include forming a slit insulating layer in the slit trench.