A method of operating a nonvolatile memory device is provided where the nonvolatile memory device includes a plurality of cell strings, and each cell string includes a plurality of multi-level cells. a voltage of a selected word line is sequentially changed to sequentially have a plurality of read voltages for determining threshold voltage states of the plurality of multi-level cells. A voltage of an adjacent word line adjacent to the selected word line is sequentially changed in synchronization with voltage changing time points of the selected word line. A load of the selected word line is reduced and an operation speed of the nonvolatile memory device is increased by synchronizing the voltage change of the selected word line and the voltage change of the adjacent word line in the same direction.