A semiconductor device including a substrate having a cell, peripheral, and boundary area; a stack structure on the cell area and including insulating and interconnection layers that are alternately stacked; a molding layer on the peripheral area boundary areas; a selection line isolation pattern extending into the stack structure; a cell channel structure passing through the stack structure; and first dummy patterns extending into the molding layer on the peripheral area, wherein upper surfaces of the first dummy patterns, an upper surface of the selection line isolation pattern, and an upper surface of the cell channel structure are coplanar, and at least one of the first dummy patterns extends in parallel with the selection line isolation pattern or cell channel structure from upper surfaces of the first dummy patterns, the upper surface of the selection line isolation pattern, and the upper surface of the cell channel structure toward the substrate.