In a non-volatile memory device and methods of forming and operating the same, one memory transistor includes sidewall selection gates covering both sidewalls of a floating gate when the floating gate and a control gate are stacked. The sidewall selection gates are in a spacer form. Since the sidewall selection gates are in a spacer form on the sidewall of the floating gate, the degree of integration of cells can be improved. Additionally, since the side wall selection gates are disposed on both sidewalls of the floating gate, a voltage applied from a bit line and a common source line can be controlled and thus conventional writing/erasing errors can be prevented. Therefore, distribution of threshold voltage can be improved.