In a 3D semiconductor memory device, a stack structure includes electrodes and first insulating layers disposed between the electrodes. The stack structure has a stair structure on a connection region. A vertical channel structure penetrates the stack structure on a cell array region. A vertical dummy structure penates the stair structure on the connection region. A second insulating layer is selectively disposed on the cell array region. A maximum thickness of the second insulating layer ranges from 1.5 times to 10 times a maximum thickness of the first insulating layer on the second insulating layer. The vertical channel structure includes an abrupt diameter change at a level of a top surface of the second insulating layer. The abrupt diameter change has a surface which is parallel to the top surface of the second insulating layer and is substantially coplanar with the top surface of the second insulating layer.