An apparatus and method for providing closed-loop transmit antenna diversity in a mobile communication system is provided. To maximize the sum of the vectors of signals received from at least two antennas at a base station, a mobile terminal calculates the vectors of the signals received from the antennas of the base station, calculates differential vectors for the input signal vectors in order to maximize the sum of the input signal vectors, and transmits information about the calculated differential vectors to the base station, which uses them to maximize the sum of the vectors of signals subsequently received by the mobile terminal.