An apparatus and method for performing beamforming or transmitting a plurality of bit streams output through Adaptive Modulation and Coding (AMC) via a plurality of transmission antennas in a base station using a multi-antenna scheme. The apparatus and method include classifying the plurality of transmission antennas into a plurality of transmission antenna groups; selecting one of the transmission antenna groups depending on a group selection signal being fed back from a mobile station; and outputting each of the bit streams to its associated transmission antenna among the transmission antennas constituting the selected transmission antenna group.