A method of programming memory cells for a non-volatile memory device is provided. The method includes performing an incremental step pulse program (ISPP) operation based on a program voltage, a first verification voltage, and a second verification voltage, and changing an increment value of the program voltage based on a first pass-fail result of the memory cells, the first pass-fail result being generated based on the first verification voltage. The ISPP operation is finished based on a second pass-fail result of the memory cells, the second pass-fail result being generated based on the second verification voltage.