In a nonvolatile memory device, a program operation is performed on a plurality of nonvolatile memory cells by programming data having a first logic state in a first group among a plurality of selected memory cells selected from the plurality of nonvolatile memory cells during a first program interval of the program operation, and thereafter, programming data having a second logic state different from the first logic state in a second group among the selected memory cells during a second program interval of the program operation after the first program interval.