A method of operating a memory device includes; applying a pre-write voltage to a selected memory cell by applying a first voltage to a first signal line connected to the selected memory cell and a second voltage to a second signal line connected to the selected memory cell during a first set writing interval, wherein a level of the first voltage is higher than a level of the second voltage, and thereafter, applying a write voltage to the selected memory cell by applying a third voltage having a level lower than the level of the first voltage and higher than the level of the second voltage to the first signal line during a second set writing interval.