To operate a memory device including a plurality of NAND strings, an unselected NAND string among a plurality of NAND strings is floated when a voltage of a selected word line is increased such that a channel voltage of the unselected NAND string is boosted. The channel voltage of the unselected NAND string may be discharged when the voltage of the selected word line is decreased. The load when the voltage of the selected word line increases may be reduced by floating the unselected NAND string to boost the channel voltage of the unselected NAND string together with the increase of the voltage of the selected word line. The load when the voltage of the selected word line is decreased may be reduced by discharging the boosted channel voltage of the unselected NAND string when the voltage of the selected word line is decreased. Through such reduction of the load of the selected word line, a voltage setup time may be reduced and an operation speed of the memory device may be enhanced.