A transmit antenna switching apparatus and method in a MIMO system are provided. The transmit antenna switching method includes determining modulation orders of users by using reception strengths of user signals with respect to each antenna; summing the determined modulation orders for each antenna; and determining an antenna having the greatest sum of the modulation orders as a transmit antenna. Accordingly, downlink performance can be remarkably improved by transmitting downlink frames through an optimal transmit antenna in accordance with channel environment.