A memory device includes a memory cell array storing input data, a clock generator circuit generating first clocks and second clocks using a reference clock, a phase information generator circuit comparing a phase of the reference clock and a phase of at least one of the first clocks and the second clocks and generating phase information as a comparison result, an intermediate data generator circuit serializing a part of input data provided from the memory cell array based on the first clocks to generate first data, serializing a remaining part of the input data to generate second data, and selectively swapping the first data and the second data using the phase information to generate intermediate data, and an output data generator circuit serializing the intermediate data using the second clocks, to output output data through one output data line.