A system comprises a nonvolatile memory device comprising a memory cell array comprising a plurality of memory blocks each comprising a plurality of cell strings, each of cell strings comprises the plurality of memory cells stacked in a direction perpendicular to a substrate, a ground selection transistor disposed between the memory cells and the substrate, and a string selection transistor disposed between the memory cells and a bitline, and configured to read stored data from the memory cells using a plurality of read voltages; and a memory controller configured to read the memory cells using a reference voltage to generate on-cell data, and adjust the read voltages of the nonvolatile memory device based on the generated on-cell data.