A method of a operating a data storage device including a nonvolatile memory device and a memory controller is provided. The method includes reading a first selection transistors connected to a first selection line from among a plurality of selection lines with a reference voltage, determining whether a first number of selection transistors, from among the first selection transistors, which have a threshold voltage less than the reference voltage is larger than a first reference value, and if the first number is larger than the first reference value, programming the first selection transistors to have threshold voltage larger than or equal to a target voltage.