The present invention provides a method and apparatus for determining the data transmit diversity mode of a base station. The present invention first estimates a first feedback weight of the mobile unit during a first slot. The first feedback weight has a first phase. Then, the present invention estimates a second feedback weight of the mobile unit during a second slot. The feedback weight has a second phase. A phase difference is obtained by comparing the first phase with the second phase. The present invention determines the applicability of closed loop transmit diversity mode according to the phase difference. Besides, if the feedback error rate is too high, the present invention commands the base station to tune off the closed loop transmit diversity for better performance.