An operation of a non-volatile memory device. A method of operating a non-volatile memory device in accordance with an aspect of the present invention, a first program operation is performed by applying a first program voltage to word lines of memory cells, constituting a memory block. As a result of the first program operation, threshold voltages of the memory cells are firstly measured. A second program operation is performed using a second program voltage, which is increased as much as a difference between a first threshold voltage, that is, a lowest voltage level of the firstly measured threshold voltages and a second threshold voltage, that is, an intermediate voltage level of the firstly measured threshold voltages. The second program operation is repeatedly performed by increasing the second program voltage as much as the difference between the first and second threshold voltages until the lowest threshold voltage becomes higher than a program verify voltage. A pass voltage is then set by reflecting a first voltage level, that is, a difference between a program voltage applied in a last program execution step and the first program voltage.