A method of operating a nonvolatile memory device includes: first programming a target transistor of a cell string of the nonvolatile memory device, wherein the target transistor has a first threshold voltage distribution after the first programming, and wherein the cell string includes a plurality of transistors; and second programming the target transistor of the cell string, wherein the first transistor has a second threshold voltage distribution after the second programming, wherein a width of the second threshold voltage distribution is less than a width of the first threshold voltage distribution.