Abstract: A data input/output circuit includes an output unit for outputting a first data strobe signal and first data in response to an internal clock generated in a delay locked loop, a first transmission line unit having a clock tree structure for transmitting the internal clock to the output unit, a second transmission line unit for transmitting the internal clock from the delay locked loop to the first transmission line unit, a duty cycle ratio correcting unit interconnected between the first transmission line unit and the second transmission line unit for correcting a duty cycle ratio of the internal clock, a data strobe signal input unit for receiving a second data strobe signal from an outside of a semiconductor memory device and generating an internal data strobe signal, and a plurality of data input units for outputting a second data in response to the internal data strobe signal.