A semiconductor memory device includes a stack including gate electrodes and insulating layers that are alternately and repeatedly stacked on a substrate. A cell channel structure penetrates the stack. The cell channel structure includes a first semiconductor pattern contacting the substrate and a first channel pattern on the first semiconductor pattern. The first semiconductor pattern extends to a first height from a surface of the substrate to a top surface of the first semiconductor pattern. A dummy channel structure on the substrate and spaced apart from the stack. The dummy channel structure includes a second semiconductor pattern contacting the substrate and a second channel pattern on the second semiconductor pattern. The second semiconductor pattern extends to a second height from the surface of the substrate to a top surface of the second semiconductor pattern. The first height is greater than the second height.