A semiconductor memory device is provided including first and second cell strings formed on a substrate, the first and second cell strings jointly connected to a bit line, wherein each of the first and second cell strings includes a ground selection unit, a memory cell, and first and second string selection units sequentially formed on the substrate to be connected to each other, wherein the ground selection unit is connected to a ground selection line, the memory cell is connected to a word line, the first string selection unit is connected to a first string selection line, and the second string selection unit is connected to a second string selection line, and wherein the second string selection unit of the first cell string has a channel dopant region.