1. An operating method of a nonvolatile memory which includes a plurality of cell strings, each cell string having a plurality of memory cells and a string selection transistor stacked on a substrate, the operating method comprising: detecting threshold voltages of the string selection transistors of the plurality of cell strings; adjusting voltages to be supplied to the string selection transistors according to the detected threshold voltages; and applying the adjusted voltages to the string selection transistors to select or unselect the plurality of cell strings, during a programming operation.