A method of operating a memory device includes changing a first read voltage, which determines a first voltage state or a second voltage state, to a voltage within a first range and determining the voltage as a first select read voltage, and changing a second read voltage, which is used to determine whether the data stored in the memory cells is a third different voltage state or a fourth different voltage state, to a voltage within a second different range and determining the voltage as a second select read voltage. The first voltage state overlaps the second voltage. The third voltage state overlaps the fourth voltage state. A difference between a voltage at an intersection of the third and fourth voltage states and the second read voltage is greater than a difference between a voltage at an intersection of the first and second voltage states and the first read voltage.