An apparatus and method for determining a transmission unit for data stream transmission in a wireless communication system in which multiple transmission and reception antennas are used and a receiver provides information on a data stream to be transmitted through a transmission unit. A fading estimator calculates fading values from the multiple transmission antennas to the multiple reception antennas. A channel quality estimator reverse-orders combined weights for multiple transmission units based on the fading values from the multiple transmission antennas to the multiple reception antennas, successively selects transmission units for data stream transmission among the reverse-ordered transmission units, determines a modulation and coding scheme (MCS) for each of the selected transmission units, and feeds back the MCS to a transmitter.