Reading methods of nonvolatile memory devices including a substrate and a plurality of memory cells which are stacked in a direction intersecting the substrate. The reading methods apply a bit line voltage to a plurality of bit lines and apply a first string selection line voltage to at least one selected string selection line. The reading methods apply a second string selection line voltage to at least one unselected string selection line and apply a read voltage to a plurality of word lines. The reading methods apply a first ground selection line voltage to at least one selected ground selection line and apply a second ground selection line voltage to at least one unselected ground selection line.