Disclosed are methods of operating a nonvolatile memory device which includes a substrate and a plurality of cell strings provided on the substrate, each cell string including a plurality of memory cells stacked in a direction perpendicular to the substrate. The methods may include applying a word line erase voltage to word lines connected to memory cells of the cell strings; floating ground selection lines connected to ground selection transistors of the cell strings and string selection lines connected to string selection transistors of the plurality of cell strings; applying a ground voltage to at least one lower dummy word line connected to at least one lower dummy memory cell between memory cells and a ground selection transistor in each of the plurality of cell strings; applying an erase voltage to the substrate; and floating the at least one lower dummy word line after applying of the erase voltage.