Provided is a method of operating a non-volatile memory device including a plurality of strings, each string including a plurality of memory cells vertically stacked on a substrate. The method includes performing an erase operation on memory cells corresponding to a plurality of string selection lines, performing an erase verification operation on first strings connected to a first string selection line from among the plurality of string selection lines, storing fail column information corresponding to a first fail string, which is erase-failed, from among the first strings, and performing an erase verification operation on second strings connected to a second string selection line from among the plurality of string selection lines, when the first strings are erase-passed.