A semiconductor memory device includes a first substrate having at least one string including a first select transistor, a second select transistor, and first memory cells connected in series between the first and second select transistors of the first substrate. The semiconductor memory device further includes a second substrate having at least one string including a first select transistor, a second select transistor, and second memory cells connected in series between the first and second select transistors of the second substrate. The number of the first memory cells of the at least one string of the first substrate is different from a number of the second memory cells of the at least one string of the second substrate. For example, the number of second memory cells may be less than the number of first memory cells.