A memory device comprises a first memory string and a second memory string. The first memory string is coupled to a first bit line and a plurality of word lines, and the second memory string is coupled to a second bit line and the word lines. When an erasing voltage is applied to the word lines, a first voltage is applied to the first bit line to erase data stored in the first memory string, and a second voltage is applied to the second bit line to set the second memory string to be floating.