There are provided a semiconductor memory device and a manufacturing method thereof. The semiconductor memory device includes: a gate stack structure including interlayer insulating layers alternately stacked with conductive patterns; a plurality of channel structures penetrating the gate stack structure, each of the plurality of channel structures with one end portion protruding farther than the gate stack structure; and a source layer including a first source layer and a second source layer, which are sequentially stacked on the top of the gate stack structure and are in contact with each other, wherein the end portion of each of the plurality of channel structures extends into the source layer, wherein the first source layer surrounds each of the plurality of channel structures, and wherein the second source layer is in contact with an upper surface of the end portion of each of the plurality of channel structures.