A memory device includes a main cell on a substrate, a first reference cell adjacent to one side of the main cell, and a second reference cell adjacent to another side of the main cell. Preferably, the main cell includes a first gate electrode on the substrate, a second gate electrode on one side of the first gate electrode and covering a top surface of the first gate electrode, a first charge trapping layer between the first gate electrode and the second gate electrode and including a first oxide-nitride-oxide (ONO) layer, a third gate electrode on another side of the first gate electrode and covering the top surface of the first gate electrode, and a second charge trapping layer between the first gate electrode and the third gate electrode and including a second ONO layer.