A method of determining a default read voltage of a non-volatile memory device which includes a plurality of first memory cells, each of which stores a plurality of data bits as one of a plurality of threshold voltages corresponding to a plurality of logic states, includes programming a first data to the first memory cells so that the logic states of the first memory cells are balanced or equally used. The method includes applying a first default read voltage included in default read voltages to word lines coupled to the first memory cells, and measuring a first ratio of first on-cells, each of which has a threshold voltage smaller than or equal to the first default read voltage, among the first memory cells, and modifying the first default read voltage based on the first ratio and a first reference value corresponding to the first default read voltage.