A memory device includes a first substrate, a first memory array, a second substrate, and at least one first vertical transistor. The first memory array is disposed on the first substrate. The first memory array includes at least one first word line structure. The first memory array is disposed between the first substrate and the second substrate in a vertical direction. The first vertical transistor is electrically connected with the first word line structure. At least a part of the at least one first vertical transistor is disposed in the second substrate.