An apparatus and method for Transmit Diversity mode transition in a mobile communication system for use with transmit diversity is disclosed. In a mobile communication system including a Node B comprising one or more antennas and a Mobile Station (MS) for transmitting Transmit Diversity information to the Node B to enable the plurality of antennas to implement the maximum Transmit Diversity. The apparatus and method for assigning weights to transmission data of the antennas to enable the Node B to implement the maximum Transmit Diversity when the mode transition occurs in the Transmit Diversity modes comprise determining whether the mode transition occurs using the Transmit Diversity information; and assigning a weight generated considering the relationship between a current weight and a previous weight used before the mode transition time to the data transmitted over the antennas if it is determined that the mode transition occurs.