According to example embodiments, a nonvolatile memory device includes a plurality of cell strings on a horizontal semiconductor layer. Each of the cell strings including a plurality of memory cells stacked in a direction perpendicular to the horizontal semiconductor layer. According to example embodiments, a programming method of the nonvolatile memory device includes setting up bitlines corresponding the cell strings, setting up a plurality of string select lines connected to the cell strings, and applying a negative voltage lower to a ground select line. The ground select line is connected to a plurality of ground select transistors between the memory cells and the semiconductor layer. The string select lines extend in a direction intersecting the bitlines. The negative voltage is lower than a ground voltage.