A method is provided for programming a nonvolatile memory device, which includes multiple memory cells connected in series in a direction substantially perpendicular to a substrate. The method includes programming a first memory cell of the multiple memory cells, and programming a second memory cell of the multiple memory cells after the first memory cell is programmed, the second memory cell being closer to the substrate than the first memory cell. A diameter of a channel hole of the first memory cell is larger than a diameter of a channel hole of the second memory cell.