A program method of a nonvolatile memory device is provided, which includes programming a memory cell in one string selected from a plurality of vertical strings; determining whether a mode of operation of the nonvolatile memory device is a pre-pulse mode; when the mode of operation is determined to be the pre-pulse mode, applying a pre-pulse having a predetermined level to a string selection line connected with a gate of a string selection transistor of at least one unselected vertical string of the plurality of vertical strings for a particular time period; and performing a verification operation on the programmed memory cell.