A nonvolatile memory device and a method of operating the same are provided. The method includes performing a plurality of program operations on a plurality of memory cells each to be programmed to one of a plurality of program states, performing a program-verify operation on programmed memory cells associated with each of the plurality of program states, the program-verify operation comprises, selecting one of the plurality of offsets based on a noise level of a common source line associated with a programmed memory cell, using the selected offset to select one of a first verify voltage and a second verify voltage higher than the first verify voltage, and verifying a program state of the programmed memory cell using the first verify voltage and the second verify voltage.