A nonvolatile memory device is operated by, inter alia, performing a program operation on memory cells belonging to a page selected from among a plurality of pages, performing a verification operation on the programmed memory cells, loading a start loop value of a fail bit count set to the selected page, from among start loop values of fail bit counts set to the respective pages, and if a loop value of the program operation is greater than or equal to the start loop value, counting a number of fail bits included in data of the programmed memory cells detected in the verification operation.