Disclosed is a method for transmitting a signal in a mobile communications system using a plurality of transmit/receive antennas. A method for transmitting a signal in accordance with an embodiment of the present invention comprises the steps of: a receiving end's calculating a signal to interference noise ratio (SINR) with respect to every antenna pairs configuring STTD pairs to be transmittable from a transmitting end; feedbacking information indicating an antenna pair having the greatest SINR to the transmitting end; and the transmitting end's performing a D-STTD transmission through the antenna pair determined by the feedbacked information. A method for transmitting a signal in accordance with another embodiment of the present invention comprises the steps of: a receiving end's feedbacking information of an antenna pair and/or an eigenvector (weight vector) of a channel matrix with respect to transmit antennas included in the space-time multiplexing (STTD) pair to a transmitting end; and the transmitting end's forming beam by multiplying a symbol with respect to each STTD pair by the eigenvector, and thereafter transmitting the beam through each transmit antenna (in case of using two antennas) or a transmit antenna pair (in case of using more than four antennas) determined by the information of the antenna pair.