A method of operating a memory device includes performing a first program operation on memory cells connected to a first word line among a plurality of word lines, performing the first program operation on memory cells connected to a second word line among the plurality of word lines, applying a turn-on voltage at a first level to the first and second word lines, applying a voltage at a level lower than the first level to a third word line among the plurality of word lines, performing a precharge operation on partial cell strings among a plurality of cell strings, and performing a second program operation on the memory cells connected to the first word line.