A nonvolatile memory comprises a memory block having memory cells stacked in a three dimensional structure. The nonvolatile memory device performs an erase operation to erase a selected sub block among sub blocks of the memory block, a verification operation to determine whether program states of memory cells of an unselected sub block of the memory block have changed as a consequence of the erase operation, and a reprogramming operation to reprogram at least a portion of the unselected sub block upon determining that at least one of the program states have changed as a consequence of the erase operation.