Provided is a programming method of a nonvolatile memory device including a plurality of memory cells. The programming method of the nonvolatile memory device includes: programming a first set of memory cells of the plurality of memory cells to a target state based on a primary program voltage such that a threshold voltage distribution of the first set of memory cells is formed; grouping the first set of memory cells into a plurality of cell groups at least one cell group having a different threshold voltage distribution width from others, based on program speeds of the first set of memory cells; and reprogramming remaining cell groups other than a first cell group that is programmed to the target state among the plurality of cell groups, to the target state based on a plurality of secondary program voltages determined based on threshold voltage distribution widths of the plurality of cell groups.